Hey davidgod,
Why are you bypassing the $7. 
$3.50 to $15 is a fair jump even if you are rolled for it. If traffic is too slow for the $7 ones, then use the ABI rule and split the $3.50 and $7.
I think it would be better to beat the $7 ones before having a shot at the $15. Every time you move up a limit your ROI will naturally drop. How far it drops is dependent on how ready you are to play that level.
Also your 3 reasons are valid.
1. Are you running significantly under Ev
2. Opponents are always getting tougher. Everyday they have a day's more education and experience.
3. Possible too, which is why you'll need to continually adjust.
I would suggest every time you move up a limit, half your tables. Give yourself a chance to adjust and as you do, start adding more tables. There are also less fish too exploit.
WTF. 100 games today at 180% ROI is a day in heaven. If you can crush $3.50s like that, ultimately you will beat those higher limits.
Keep Crushing
Laz