i hate to tell you, but you cant decide to 'not' share the vette with your dad now, you are a co-signer so its too late. why buy a slower ls1 car, and get rid of the gtp? the vettes are nice, dont get me wrong i would like to have one, but you have access to drive one if you get the urge, and you have a quick car already. a z06 would be a different story power wise, but i still say keep your PAID OFF car, and use it for a street/strip car and just buy a cheap dd to drive this winter.
which brings me to my next point, its not my place to get into your personal life, but i just read a post saying you dont have a job? wtf? you cant possibly be thinking of spending 15-25k on a car without having a SOLID income. regardless how much money you have in the bank ect...
finally, keep the gtp, keep the xp cam for now. you can make plenty of power with that cam. im sure you can take it to mid 11's/350whp. only reason i saw this, is before i know you arent doing the work yourself, and it will cost a good amount of money to change. think of it this way:
heads: not sure on money, 1500-4k?
trans: 3-4k?
ic: 1k
should put you close to 11's i would guess. with the right tuning, and weight reduction/supporting mods im sure mid 11's could be achieved.
so instead of loosing 6-10k on demodding/sellin the gtp, and spending 15k+, you can spend around 6-8 and be done with it, and have a reliable street car that will tear up most cars you'll come into contact with on the street. plus you will save your ass on insurance rates as well. just my .02