Believe me, MOS on Pegasos feels a lot faster than UAE on any PC, gigahertz CPU or not. Pure emulators just don't come close.
Below are the compilation times of a program (GoldED) compiled with SAS/C under UAE, UAE with JIT and MOS without JIT. Compilation is a reasonable test for raw performance. Integer performance obviously. The compiler is run with precompiled headers to take HD speed out of the equation.
UAE 0.8.17r3 is used, which is not the latest version, for obscure personal reasone: it works better with a tablet. The PC@1.4 GHz has 512 MB DDR-RAM, 40 GB Maxtor HD and 160 GB RAID; Maxtor used for test. The Pegasos@600MHz has 256 MB SD-RAM and a 40 GB Barracuda IDE.
UAE /w JIT: 1: 59 min
UAE no JIT: 11:52 min
MorphOS 1.3 no JIT: 4:19 min
Given those figures, the PC with UAE+JIT is about 2.5 times faster than MorphOS. However, the PC's CPU is also clocked 2.3 times faster. A crude guesstimate would be that a Pegasos/MOS1.3 and UAE/JIT are running emulated code equally fast on CPUs with the same clock frequency.
Now the interesting part: JIT massively contributes to the speed of UAE. JIT makes UAE compile seven times faster. If JIT for Pegasos (which will start shipping with MOS1.4?) has an effect anything like that, it is quite possible that a Pegasos at 600MHz will, with respect to emulated code, meet or exceed the performance of very fast PCs.