The stock block/piston's are good for something like 650rwhp anything past that i would upgrade the piston's for reliablity but then again i doubt your car will be seeing high boost all the time so you will be fine . I have seen some na-t member's make more hp then 800hp at the motor on the stock block but who knows how long ther motor's are going to last Mostly everyone who goes na-t just swap's out the headgasket as its alot cheaper and does the same job basically as new piston's of lowering the compression.. The lower compression the higher the boost you can run
Its just cheaper and easier! The ONLY difference between the NA and TT blocks is lower compression pistons and oil squirters. So if you switch to low compression pistons, you damn near have a TT block. Ive been told by several top companies like BL that the NA block is good for almost 900 hp which is right around the same as a TT. The crank and rods are the same as TT. I say save the headache and just do the HG cuz EVERY kit making high hp does.
If you're going to break apart the motor and you have a little extra dough then go for the stock or built TT pistons. It all depends on what you want and what your ultimate goal is.
If all you're doing is slapping on a turbo and your compression is good (check it before you tear down, obviously) and you're not planning on more than 700 HP then just get a thicker head gasket. Granted you may pull more than 700 HP out of your stock NA block, but if you're going for big big power then you might as well spend the extra couple hundred and build it up.