Originally Posted by
Gnomes2169
Fluff wise, the rune mage is filling its runic grafts with magical power that remains relatively stable while bound to the rune. However, in times of crisis, the rune mage may "unleash the floodgates" if you will and temporarily break their rune/ drain their magical stores to make the next casting more powerful. It's basically a trump card or panic button, if you will, set ahead of time to give the "academic" sorc an edge in a future engagement.
That's what Metamagic Runes do. Instead of being something to spend sorc points on, though, they allow you to save them when casting certain spells (from your grafts), which in turn gives you a bit more leeway in how you can sprnd your sorc points. I don't see any need to give it another method of influencing its sorc points.
The rune mage doesn't have 2 level 8/ 9 spells to cast per long rest, actually. It has 2 of each level known with one of them being prepared and thus interchangable, but that's about it. If you are thinking that reckless power gives you an extra slot of a higher level, well... Read the end again. It still expends a spell slot, and it has the hefty price of not being able to prepare grafts of the level you just cast for a week (unless you are level 18... Then it's only 3 days).