Background info: 1997 I30, 113k miles. Oil is Mobil 1 synthetic 5W-
30 changed regularly. Transmission was serviced 5,000 miles ago
(power flush, new fluid, etc.) The butterfly plate was cleaned about
5,000 miles ago as well, airfilter looks great.

PRoblem: Car shifts fine during regular driving. About 1.5 weeks
ago, I was going 25mph and I floored the vehicle (vehicle was warmed
up) and when I hit the gas, it felt like I hit a curb! The whole car
jerked and made a loud boom. Car continued to drive fine after
that. My mechanic advised me to reset my ECU (ECM?) by disconnecting
my negative battery terminal for about 20 mins and then drive the car
to operating temperature and see if the problem happens again. For the
next 2 or 3 days, I had no problems. However, a couple days ago, I
had family in the car and we were merging onto the interstate. At 45
mph, I floored the pedal as a semi was coming up onto the merging
lane very fast. Sure enough, when I floored it, the car sounded like
it hit a curb going 45 mph. My mechanic suggested that the
transmission mount rubber is probably busted, and that when I floor
the vehicle, it's enough force for the transmission to be slapping metal-to-metal because the rubber is busted.

Does this sound like an accurate diagnosis? My car is still driving
completely fine under normal operation, but it's bugging me that when
it's floored it made that loud noise/jerk twice. I just had my
tranny serviced so it's worrying me that it's doing this.

 

It definately could be your mounts. Weather they are the tranny or motor mounts I dont know. I dont know if you do any work on your car, but you should climb under and take a look.

Im not sure if the mounts are too different from A32 to A33, but I imagine someone here might be able to help you find them. You may also try the 4th gen. Max forums too. There will be a lot more people there so you have a better chance of someone understanding your situation.