As far as you orginal question, here's my take on it:
1) Upgrade MAFs when you peg the stock MAF and that's around 300 RWHP or so. IMHO there is nothing to be gained by going to an aftermarket MAF and there is always the risk of worse driveability.
2) You need to size a MAF on ELECTRONIC capacity, not necessarily airflow size. Rarely is a MAF a restriction in the system. I usually recommend another OEM style MAF when upgrading something like a 90mm LMAF. You should size a MAF so you are using 80% of it's electronic range so you end up with good low end resolution.
3) See above
4) Grady and I usually agree 99% of the time on everything - but on Pro-M we part company. That's because I see tons of them when I tune cars and doing that has made me hate them. I understand the new company, Professional Mass Air Systems (they took over Pro-M's equipment when Pro-M went belly-up) really does a good job, so I look forward to seeing a bunch of their MAFs on cars. If you get a 90mm LMAF, you need a tune or a TwEECer or other tuning device. One would think that you would be OK with a 'calibrated' MAF, but you STILL need a tune or the load calculations will be off - I have some info on this on my website - see sig. Look in the tech articles on running a late model MAF in an earlier model car.
5) See above
As far as the lean condition, how do you know it's lean? How do you know it's running rich part of the time? And if you have a wideband, what numbers are you considering lean - 14.64 is normal at idle and part throttle. A lean motor will surprisingly smell rich to some people - a wideband is the only for sure way to know. If you are getting a lean code, look for several things - the obvious are vacuum leaks ? (hoses, upper/lower, etc.) the not so obvious are exhaust leaks ahead of the 02 sensors and air leaks in the TB to MAF tube, although this will usually also trip a MAF voltage below min code.
Good Luck!
Don