That would've been enough for the X-33
The X-33 wasn't designed to go into orbit. Instead, it was intended to go somewhere around Mach 12, if memory serves. 1800 degrees is probably enough protection for that speed, so anything more would've only added to the weight and cost.
As an aside, my brother Steve made a lot of the aluminum parts for the X-33 oxygen tank (the one that worked). It was a very complex shaped tank, but then he's a very good machinist and welder.