I'm not finding the Monster iDrive... got a link?
Regarding the fit kit from Crutchfield... Crutchfield.com doesn't even list the '07 GTI as an option. How did you order a kit?
its the same as the 2006 new gti/jetta. I highly reccomend adding an amp to your alpine head unit to power the crossovers for the speakers. you dont need anything big a 4x40watts @4o will do you fine.
if your going to put in new speakers it gets a little harder there isnt many companies that make 3 way component sets. you have a 6.5" 4" and tweeter in your doors. and a 6.5" and tweeter in your rear. I went with mb quart to get true 3 way set in my door and the sound is insane. mb quart pce210 with an add a woofer[QSC 164] for your door,mb quart RVF 216 for the rear and you'll be very happy with the sound. you can also just get a 6.5 and run it as a 2nd mid in your door if you go with someone that doesnt make 3 way sets i did that with my infinity speakers but blew the 4" speakers fast doing so.
its all a matter of how much you care about sound/price i spend alot of time in my car and need to have a nice sounding system to keep me happy. I also need full control over my ipod from my head units, in total i spent about 1,500.00 for my system if you count my sub amps and head unit into the mix.