We have stopped at 2. Partly I feel too old at nearing 40, partly I hated being pregnant and mostly because I haven't had a full nights sleep in 5 years and I have a mood to match it. I think HG dd and perhaps HG ds would have slept better if they were normal on the curve, I think they might have been less demanding, who knows but I guess the intensity means I can parent 2 kids well any more I'd be making compromises I'm not happy with. I guess that calls in the giftedness...
I would say though that I did resent ds for a while taking me away from dd, then dd for a while for not letting me get to know him. It took a while for me to get the balancing act right but now I would have our circus any different ( besides which eventually they distract each other!!! Bonus)