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Putting 20" rims on my Cadillac. The stock rims are 16" tires are 225/60-16 and I'll be putting on 20" rims with 245/35-20 tires. The MPH difference of the speedometer after changing between the two is less than 1mph (ie; Speedometer shows 70mph, I'm actually going 70.320mph; Speedo shows 60mph, I'm actually going 60.274 mph) and the overall diameter isn't much difference. From touching the ground to the top of the tires, the 20's are only %0.46 bigger equaling out to 1.55mm higher than the 16's would be.

I know the 20's are gonna be a bit heavier so I'll have slightly less power and the ride will be a little bit bumpier, and the brakes and suspension are already taken care of to compensate for the 20's.
So heres my question, should I be worried about my engine/tranny at all?

not at all. you did not make any great change in the speed of the vehicle.
The engine and tranny will feel no change
You as the driver on the other hand will notice a different ride and handling.