My experience with an Extreme (30mm) chassis was quite good. Yes, it is “cheap” in price but I can’t say that there was anything specifically “cheap” about the design and/or build quality. The only thing that I didn’t like was when the one I used was assembled it didn’t have foot rests (not sure if this was the lack of being supplied with the kart or an omission during assembly) but this was fixed with $30 worth of foot rests.
For reference, I rented this kart for a regional 4 cycle series (I’m 6’0″ 185lbs) and as far as performance, I was just as fast in this kart as I was in the CRG I had used at the previous event (only 0.03 seconds of the leader who was running a CRG and had lots of laps under his belt). I have always run CRG karts in both TaG and shifter (I currently run a shifter as my primary class and the 4 cycle was just to have some fun) so this is the only brand I can compare performance with. It is a long story how I ended up renting an Extreme chassis but I came away pleasantly surprised and in my opinion, is worth a serious look for someone who wants a new kart but doesn’t have the budget for one of the bigger brands.