The blocks can withstand well over 1000hp. The newer motors from '08 on, have forged internals (Cobra Crank, Manley Rods, Forged Pistons). That's the reason you see so many cars with Blowers, Turbos, and Nitrous, cause these cars can take it.
The Automatic 5R55 Transmissions are very strong and can back up whatever you throw at it due to it's larger than normal and hardened 300M input shaft. The only reason people switch to the 4R70W Trannys is because they have a Transbrake. Other than that, they're equal in comparison.
The 5 speed Manual 3650 Transmissions are the equivelant to the TKO500's and can really take a beating. A lot of racers in "Real Street" Class use the 3650's due to ease of shifting because they come straight from Ford with Face Plated Gears which make "Power Shifting" very easy. So hold the gas to the floor and let her rip. The main reason why people switch to the 6060 Transmissions from the GT500's is due to the fact that they can have a 6th gear for highway cruising. Other than that, the 6060 is....okay.
The Rear ends of these cars are almost as strong as a Ford 9''. Most of the guys that switch to a 9'' do so only for braggin rights to say "Hey, I have a Ford 9'' in my car". They're good but not necessary.
If you happen to do a search on any of the above mentioned topics, you'll be able to prove/disprove the information I provided you with. I had a little more free time this morning than normal to mess with you.