[3.5] New campaign setting alternate Plane Shift rules, please review

As the title suggests, I'm designing a campaign setting, and its structure requires changes to planar-travel spells. I wanted to run these here to see if the mechanics seem workable.

Basically, there's only one plane accessible to mortals besides the Material: the Shards. It's arranged into 6 layers; for simplicity's sake the top layer is Layer 1 and the bottom is Layer 6. Cosmology-wise, the Material Plane is effectively "layer 0." I'm aiming to make a fairly hostile spirit world, with the bottom layers being the most difficult to reach and the most physically dangerous, except for Layer 6 which is the stronghold of the mortal-friendly deity group in the setting (hard to reach, but safe).

Plane Shift: Cast anywhere in the Material Plane, lets the caster travel to Layer 1. A casting anywhere in Layer 1 can return to the Material Plane. Going deeper requires casting Plane Shift at certain nexus points where two adjacent layers are closer together; i.e. a nexus point in Layer 3 could lead to either Layer 2 or Layer 4, not both. Going deeper than Layer 1 requires a caster level check, DC 15 + 5 per layer beyond 2 (DC 20 to enter Layer 3, DC 25 to enter Layer 4, DC 30 to enter Layer 5, DC 35 to enter Layer 6). No caster level check is required to go up a layer.

Greater Plane Shift: As Plane Shift, but no caster level check is required at nexus points. Additionally, it can be used to jump layers from any location, not just nexus points. Travelling this way requires a caster level check, DC 25, to succeed. The caster can even jump to non-adjacent layers, but doing so always requires the caster level check and adds +5 to the DC for every layer skipped (Going from Layer 1 to Layer 2 is DC 25. Going from Layer 1 to 3 is DC 30, and layer 1 to 4 is 35, etc).

• Failing the caster level check for either Plane Shift or Greater Plane Shift causes every target of the spell to take 1d6 points of untyped damage for every 5 points by which the check failed, rounded up, as the planar structure of the Shards slashes into the targets' being. This damage cannot be resisted or prevented in any way. Any creature killed by this damage is shredded into bloody ribbons, rendering Raise Dead (but not Resurrection, etc) useless.

Gate: You can go anywhere on any other layer with no effort beyond casting the spell. It's 9th-level, baby.

So do these mechanics work out, or should I adjust the DCs?

First a bit of number crunching on the success rates. This doesn't take into account various CL booster, but whatever.

Planar Shift
{table=head]Layer|CL9|CL10|CL11|CL12|CL13|CL14|CL15|CL16|CL17| CL18|CL19|CL20
|(Cleric)||||(Wiz/Sorc)
1 to 2|70%|75%|80%|85%|90%|95%|100%|100%|100%|100%|100% |100%
2 to 3|45%|50%|55%|60%|65%|70%|75%|80%|85%|90%|95%|100%
3 to 4|20%|25%|30%|35%|40%|45%|50%|55%|60%|65%|70%|75%
4 to 5|0%|0%|5%|10%|15%|20%|25%|30%|35%|40%|45%|50%
5 to 6|0%|0%|0%|0%|0%|0%|0%|5%|10%|15%|20%|25%
[/table]

Greater Planar Shift
{table=head]Layer|CL13|CL14|CL15|CL16|CL17|CL18|CL19|CL20
|(Cleric)||(Wiz/Sorc)
1 to 2[br]2 to 3[br]3 to 4[br]4 to 5[br]5 to 6 |45%|50%|55%|60%|65%|70%|75%|80%
0 to 2[br]1 to 3[br]2 to 4[br]3 to 5[br]4 to 6 |20%|25%|30%|35%|40%|45%|50%|55%
0 to 3[br]1 to 4[br]2 to 5[br]3 to 6|0%|0%|5%|10%|15%|20%|25%|30%
0 to 4[br]1 to 5[br]2 to 6|0%|0%|0%|0%|0%|0%|0%|5%
0 to 5[br]0 to 6[br]1 to 6|0%|0%|0%|0%|0%|0%|0%|0%
[/table]

So what does this tell me? Planar shift pretty much does what I think you want it to do, so we're good there. Greater planar shift has a couple of problems. The "no check while you're at a nexus" part is fine; though you're already auto-succeeding while going from 1 to 2 (and possibly 2 to 3 with boosters) with the original spell, it's a notable difference when going through the lower layers.

The "shift from anywhere and possibly skip layers" part, however, doesn't function too well. First, if you're a cleric, it's not really usable once you get it, due to the low success chance. If you're a Cleric 15 or a Sorcizard, it's slightly more usable, but at that point there's only a two-level window between it and Gate. Once the player gets Gate, and they have a choice between using an eighth-level spell with a 65% (or less) success chance or using a ninth-level spell with a guaranteed chance, they're going to go with the ninth-level spell every time. Second, the chances of skipping any more than two layers are pretty much nil. Maybe this is what you want, but at that point why keep that aspect of the spell in?

Three options as I see it:
1) Modify the DCs. 20 + 5 per layer skipped means that at CL20 you can GPS to the next layer guaranteed, or skip a layer 80% of the time.

2) Make the DC based on the destination layer, rather than the number of layer skipped. 15 + 5 per layer past the first means you always succeed in going to the first once you get the spell. At CL20, that's auto-succeed at reaching the second, 75% to reach the third, and 50% to reach the fourth. Past that, you should find the nexuses, which lets you take advantage of the automatic nexus travel aspect of the spell.

3) Remove this aspect of the spell. The automatic nexus travel is still a great improvement over plane shift, especially since the players will probably be delving on the deeper layers at those high levels. It would also make locating nexuses more important, which would provide another source for adventure and allow you to make use of some of the portal spells from Spell Compendium and the Planar Handbook.