Is there an upper limit on the amount of hash rate for Bitcoin's Proof of Work? Is there a big difference security-wise going from 175 EH to 275 EH?
AFAIK there's no limit set by the Bitcoin network protocol. It obviously affects the network difficulty / target but I believe the upper bound of difficulty represents an extraordinarily high hashrate.
What is the maximum difficulty?
There is no minimum target. The maximum difficulty is roughly: maximum_target / 1 (since 0 would result in infinity), which is a ridiculously huge number (about 2^224).
The actual maximum difficulty is when current_target=0, but we would not be able to calculate the difficulty if that happened. (fortunately it never will, so we're ok.)
That link also shows you how to convert that maximum non-infinite difficulty number to a hashrate. I think that gives you around 5 x 10141 (an exahash is a tiny 1018)
The difference in security between 175 and 275 EH/s really depends on the available amount of unused hashpower as much as on the deployed hashpower.
Note that hash rate is measured in EH/s not EH.