The issue with too large of a pump is the fuel lines. If you supply too much fuel that the engine doesnt need, you create additional restriction that manifests itself in the form of higher pressure at the rail. Now, one can simply adjust a fuel pressure regulator down, but if you dont have one, then you are kinda stuck.
This is more of an issue if sticking a 255 hi flow pump on a stock 5.0 engine and likely not as noticable on an H/C/I car.
WHen i went from the stock 88lph to a 190lph on my stock engine, i had to adjust the regulator down from the previous setting as far as it could go and it still a psi or so too high. Hopefully the latest HP bump takes care of that.
Will be different for everyone, but I tend to suggest to size your pump appropriately. A stockish cruiser that will never get modded does not need a 255 pump.