1. No comparison, both preamp mods sound better than the stock.
2. A/D is suprisingly good, I ran v2> stock ua5 for a few shows and was quite pleased with the results. Having limited experience with the ADC20, I can't really say if it would be any better/worse.
3. UA5 stock sounds warm, a bit grainy, slightly rolled off extreme highs & lows, a little noisy at high gain. Overall a pretty laid back presentation, which I found colored but enjoyable. Much better to be warm/dark sounding than overly bright like some of the less expensive preamps.
I'm not sure if the stock ua5 would be an improvement over the mp2>adc20.