I haven't done it yet, but I'm planning to on my '92 9000 Turbo this year (er possibly next winter).
Megasquirt is it's own engine management system. It will need to be tuned for your car.
You can raise the rev limit through Megasquirt, however I'm not sure if it's going to help with your application.
I'm not sure about HP gain with a non-modified car, plus every car is different.
It is doable, I mean seriously... Anything is possible.
Here's a thread that goes into detail on installing MS on a LH 2.4 car, if I remember right it dives into installing it on a 900, however the 9000 would be very similar.
http://saablink.net/forum/showthread.php?t=16259&highlight=megasquirt
It might help with some ETS issues if your car has ETS, however you could always swap off all of the ETS components, and that would be easier and keep your car running well in stock form.