I am not so sure what I think of messing with the HP system. HP are the bread and butter of combat, so any increase in HP is a big increase in power. While such systems are good at low levels, by 7th or so they really start to make their affect known. I've seen 8th level fighters break the 100 hp mark with only an 18 con, and that most definitely changes how a group interracts with CRs.
Another way of looking at it... although Toughness is considered one of the weaker feats, it is still a feat. Most HP systems change the average on the d10 by upwards of a point and a half, meaning that every other level you are giving a free (albeit weak) feat.