It’s capped for all levels afaik but people have said there are exceptions possible. Sign on is over first two years and it is to make up for the backloaded vesting schedule for your initial RSU grant which vests over 4 years (5/15/40/40)
Exceptions are not possible. You can get additional cash, sure, but it won't be called base. There are a few people with grandfathered in numbers that are slight variants (SVPs who have been here for 15-20 years make make 175k in Seattle or so).
Jeff Bezos is at L12 and SVPs are at L11, which can have more than 160K base as exception to the rule (besides large sign on + stock options). I have not heard anyone at L10 or below getting more than 160K base in Seattle location.