Please... You're talking to a guy who can roll the arcade game.
Practice at 2600 BnJ doesn't do any good because you can't learn
the jumps since THEY'RE NOT CONSISTENT!
Right! The jump is X meters... So you need to be travelling at a rate
of >= Y meters/s to make it. Only Y seems to vary. Sometimes 140 will clear a jump, but other times you need to be going at least 160. It's not consistent. Wouldn't be some bad if it were just a gap, but island jumping assures death with unpredictable jumps.
No, that you CAN learn with practice, which is why I'm (even to this day) pretty good at the arcade version. Heck, I wasn't too shabby at the NES version either, though I used the continue code to go all the way to the end rather than spend weeks of practice in getting there.