Complex endorsement question

I already have a high-performance sign-off with several hours in a
C182 and a couple in a Bonanza. I have found a couple of places which
rent a C172RG for a reasonable price. My goal is to be get my
commercial and I'm starting to work on my IFR. With that in mind, I'm
looking at taking at least part of the IFR training in the 172RG.

How many hours does it normally take to get a sign-off and what
training will involved in getting it? To ask it a different way, what
differences will I need to learn other than the the max gear down
speed and remembering to lower the gear? I know the high performance
retracts means flying a more slipperly airplane which means the need
to plan altitude and speed for the landing a lot further in advance
than a fixed gear.. (at least that was the case with the Bonanza).

thanks,

John