A little extra will always matter...we've got crap roads here in Belgium, too (not all, but many), as well as a fair amount of cobblestone and pavé. I've ridden 28, 30, 32 across all of this. Bigger is always more comfortable, and not necessarily "slower." Check out this link from SILCA for determining pressures, I use it, and have never had an issue, I pretty much don't care what is printed on the sidewall because it doesn't take into account all the factors at play such as total system weight, surface condition, actual measured tire width, etc.