Just because it's a cooktop vs. a range doesn't mean they are different species. They'll still have much of the same internals.

It's not that downdrafts are "not acceptable". It's not opinion based. It's fact based. You're attempting to contravene the laws of phisics. When you have a dragon in the kitchen, it produces way too much heat, steam and smoke to ever be captured by any downdraft. If you have a fan strong enough to suck all of that rising stuff back down, then it's strong enough to suck on the flame and potentially put it out. It's certainly going to be LOUD and marginally effective no matter what.

I owned a 36" 6 burner Viking cooktop (drop-in) from about 2000 to 2007. My personal opinion is that the design of the sealed burners on the viking produces very uneven heating around the perimeter of the pan. The large burner was almost completely unusable because the flames came out around 12-14" in diameter. Our largest saute pan which was 14" only got heat around the very edges. My 16" wok was only hot from about half way up the side rather than at the bottom where it was supposed to be. Also, I found that the aluminum collar around the burners became horribly stained even though I was very careful about cleaning after each use. The the heat right around and under the burners was intense enough to bake on any spatters almost immediately onto the aluminum collar or the stainless steel around the burners.

My advise would be to find a live unit somewhere and try using it paying particular attention to heat distribution and such. As long as you are happy with your decision, then that's all that really matters.

Good luck.