engine noise and knock on a '16 'test drive' - a couple questions...

Hi, I've already gotten good input here about my concern over the CUE system as I am looking
very hard at a late model ATS Coupe, '15 or '16. I've test-driven a few, most recently a '16 2.0T,
one-owner trade-in at a reputable local dealership with only about 21k miles on it.
Price is pretty decent at around 21k.

Last weekendI dropped in to test drive it. The salesperson had to throw some gas in it for me
first. My impressions were these. Engine noise was noticeably louder than other ATS Coupes I'd
driven - and louder than any Cadillac should be IMO.
The major thing though is that it seemed like it wasn't downshifting enough to keep the engine
from lugging. So, overall the shifting didn't seem as smooth as I'd expect. I don't think this
was a transmission issue. The next day i started reading up on the forums about the fact that this
model requires Premium gas. I assume that whoever ran out to fill it first just used Regular.
(This is not on a Caddy dealership lot FWIW). For that matter, have no idea if the previous owner
routinely used Premium - and on that point, whether not having done so could cause any
significant damage to the engine?

Later the same afternoon, I test drove a '15 ATS 2.0T at another dealership.
I found that engine noise way quieter, and the shifting very smooth and normal.
I realize this '15 is a 6 vs 8-speed and frankly i thought it was a much more satisfying
experience than that of the '16 ATS. If the '15 didn't have triple the miles on it
i might've made an offer on it.

So i guess this all boils down to whether the gas thing could've actually been a factor in
how the '16 ran?

Meantime I've asked to take it for another test drive if they will put Premium gas in it for
me and if they aren't BS'ing me - and actually want to try and sell a car, it supposedly will
be ready for me to do that on Saturday. Will they bleed out all the Regular beforehand?
Maybe not: )

I welcome and appreciate any thoughts on this and thanks for your time!

Mike in Mich