I thought we talked about a 150 going all the way to 240+ bhp. This is obviously a pure nativity. 204 (199 to be exact) getting 20% increase sounds plausible on the other hand.
Again, soft bump only for 150 bhp would get you up to 170 (165 more likely) bhp region. Going beyond would put a lot of strain on the engine without any doubts.
Our second hand T6 came with ABT chip fitted from new. I believe here (in Switzerland) the VW commercial dealers even offer it as an option when speccing a new van. We have the 204 ->240ps version. It drives very well, except from cold, when I lift my foot off the brake to crawl out of a parking space for example it quite often will stall. I haven't had much time in a non-remapped version, so I don't know if this issue is specifically as a result of the mapping.
I remapped my old van from 102 to 666 and it went like the devil.
But seriously, never heard of under-warranty remapping. Sounds bogus. I know a guy who did 140 up to 190 remap of his T5 (he's a big kid), but I wonder what the point is. It's a van, a brilliant modern van, at the end of the day. 140 is great and responsive as it is.