This board gets me in trouble sometimes
towers are troublesome, especially those that tilt up and the opinions out there are all over the place.
8" diameter 12 gauge tubing is very rigid stuff and very much up to the job of supporting a 12' wind turbine if properly guyed. (6" would do it fine too so there is some safety factor here)
a 45' gin pole, for a 90' tower is reasonable, and the longer the gin pole is... the less the forces are actually.
I've seen towers of this type with as little as a 20% guy radius (they require large anchors) - here we started out talking about perhaps 35% and now it looks like he can do almost 50%. It'll be fine I expect. I might be a 'hillbillie' ;-) - this tower is almost the same as mine (mine is from 10" tubing but has a 20' turbine perched on it, it's very rigid) (we beat 100mph winds up here last week with the machine running... everything is fine)
We've put a few 12' machines on 6" tubing and they've been fine (they are also very rigid)
I've seen some very heavy proven 12' turbines perched on 4.5" sched 40 pipe.. some of those towers failed.
This place is a great forum to share information but sometimes the 'misinformation' and 'bad ideas' I see put as 'facts' scare me....