Back in 2011, I was doing the inverse where I connected my MBA in Target Disk Mode for my iMac to boot from. It was like a fancy external display that also provided excellent performance. Rather exotic, but no headaches of maintaining two installs, synchronizing files, and so on. A fun anecdote, but nowadays there is no reason to overcomplicate things. To use the MBA as a screen, you'd need a capture card. Hard to recommend that.

If you're really worried about throttling, at home you could put the MBA on a cooling stand. Naturally that only makes sense if it's generally fast enough for you. Otherwise just get the MBP, since a couple of pounds are not worth the hassle of having two machines. Do consider offloading some work to the cloud.