what kind of speed could arm jit produce ?

[Henrik]

The same CPU at higher clock will always be faster than the same CPU at lower clock

Comparing clocks across different CPUs might not always be meaningful. An i7 at 3.6GHz is many times faster than a P4 at 3.6GHz, and it's probably similar between mobile CPU. Look at benchmarks, high single core CPU scores will likely mean faster PPSSPP.