I once added the weight (including the spare) at somewhere around 250-300 lbs. And I've had this rack in a similar incarnation for the last 2-3 years, so I doubt it's going to fall apart anytime soon. The crossbars are a little bent (maybe 1/2" deflection at center), so I might add a third crossmember to distribute the weight, but why?
As far as the gutter mounts go, I haven't seen them peeling off the side yet, and I check every time I take the rack off. I do that to fit the Jeep into the garage (2" clearance), and unloaded it's light enough for me and my gf to pick up and off - much more convenient than bolted to the roof, plus I can make changes to the design easier.
It has a small effect on gas mileage. I just drove Chicago-PDX loaded w/2 bikes & tons of camping gear. Best mileage I got was 15.5 mpg, worst was 12.5 (in Neb where it was windy). But last year, loaded w/kayak & bike & camping gear, I got 17 mpg over the cascades. So your guess is good as mine.
I thought it would be very top heavy, especially since it's 8'2" to the top of the coffin. Last summer, on a trip to the beach (w/ just camping gear & 1 bike), I got cut off on the freeway in portland. Slammed the breaks, swerved left, skidded 90 degrees to the freeway, thought we were going to die, but never tipped. But I don't drive like a maniac, so that's the only experience I've had.
Hope this helps