For most printed monsters and most levels, an efficiently played T1 will have:
1) A 100% chance of winning an equal CR encounter
2) The ability to completely solve many encounters per day, by using the right spell. Potentially, they can win as many encounters as spells they have available, since they have access to a huge amount of win-buttons. Some of them (druid) could go on for dozens and dozens of encounters before they are out of hp.

With the premise that the CR system is very broken and malfunctioning, you want to look at T4-5 for that kind of performance. That is the power level the 3.5 testers played at, so it is balanced there.