Just wondering what approximately what mods it would take to make a GTP run in the 11's. Also, about how much HP and WHP would you be pushing in order to achieve this?

You would need a big turbo:

https://www.zzperformance.com/grand_prix/products1.php?id=411&catid=109
-You would upgrade the turbo and get an intercooler and headers with that kit.
-Cam
-Maybe even heads

-About 500 HP, and 550 lb/ft to get well into the 11's

As it says in the article, that turbo will support a 10 second 1/4 with few additional mods. If you wanna get there fast, just throw a big turbo on it.
Here is a cool video demonstrating what you will need:
http://www.99-gtp.com/images/Videos/Daves_Turbo_Final.wmv

Start with the XPZ/IS4 cam and some ZZP/intense stg. 4 heads, on top of that, you're gonna need enough supporting mods to run a 2.55"-2.7" pulley, which don't ask me what those are, but I'm sure that a Stage 3 intercooler would be necessary. Also a GenV M90, because they are so much more efficient at making boost, would be a good idea to keep KR down.

lots. Turbo/little pulley/heads/headers/intercooler/

I don't know if a blower would get you in the 11s. If not turbo, a whipple or novi are probably your only options.

M90 with a ton of supporting mods can do it, but going turbo would probably be easier and have more potential.

There are plenty of M90 cars that have broken 11's. It's just hard to do, and you have either be all in, or all out. No mild modding allowed.

Novi with huge intercooler, cam, and headers.
(I want a cam and intercooler )

Just shoot for twelves... it'll be easier and cheaper and you can use your gen3 M90.

Don't forget to say bye bye to your transmission!

I'm tellen ya, if you are serious about getting into the 11's all you gotta do is turbo it, cam, heads.

Oh yeah and your gonna need a tranny. Its not gonna hold up with 500+WHP