With our return style fuel system you will notice no differences. By upgrading to a 255 now you save yourself the trouble of upgrading again in the future.
The only potential down side that people have debated before (which I feel is negligible) is with the extra fuel moving around it warms up more than with a lower volume pump.
The HP pumps just flow better at higher pressures than the non-HP counterparts.
It will not hurt mpg, yes fuel pressure will go up. When I installed mine I jumped up 3 psi in fuel pressure. That is with stock heads and cam wit a 255lph.