hey man, um with the size of the speakers in the gtr im not too sure. but just get urself some spacers made up, they are REALLLY easy to make and this way u can fit wat ever size speakers u want, preferably 6.5"
with the "best coaxial" on the market try post this in http://www.mobileelectronics.com.au. um also because audio is a very subject and personal choice might be best to go and hear some speakers urself at an audio store. this way u can judge urself what speakers are better.