I tried simulating with high speed opamps, the best I used was the ADA4817, it's really nice, but you end up with ringing artifacts at the switching point, and a small square wave at the inverting node. All of these are distortion artifacts. It's pretty good but not as good as what I wanted, so I went open loop for the triangle generation. With the configuration I mentioned you get super clean rectangular current pulses that are very flat i.e. low distortion. The waveform looks very clean. It all depends on how you deal with the non linearities of the circuit. It's the detail that's important, how you buffer the triangle wave in the open loops case is super important.