I have come very close to getting a 16 second time in "A Downhill Battle". If this where possible it would untie the world record. are there any good Super Mario Maker 2 Speedrunners\TASers that know about slopes and other speed tech in NSMBU?

by this point, it seems extremely unlikely that it is possible, but idk

I have gotten like 17.2, but I don't think 16 is possible without some corner boosts or some other hard tech

I think sub17 is impossible. I got 17.121 on almost perfect slopes. It would be difficult to save 6 frames with corner boost.

Unless something is discovered to save a few frames, I highly doubt 16 will happen. But who knows.