This means that for most characters (I think most players consider Con an important stat but not the most important, and thus it will likely be in the +1 to +3 range), Sturdy is as bad or worse than Toughness.
Maybe I'm missing something, but it seems Sturdy is as
always better than Toughness, and sometimes significantly so. I think I personally like the variant suggest above, where it stacks with your Fort Save. Another option would be to model it after "Mind Over Body" from FRCS, allow double CON for bonus hp. This means a first level dwarf with 20 Con could take a feat for 10 (!) bonus hp. Of course for the first level elf wizard with 12 Con its only 2.
That seems to be the problem with modifying Toughness. Toughness is excellent if taken at first level and you are in a class with a d4 or d6 HD. You are getting at least 50% more hp for 1 feat. For higher HD it is less useful, since it is a smaller percentage. Again, those 3 hp will keep mattering to a wizard (or a multi-wizard) as Andy Collins (I think) suggested, if you keep track of how many times you go down to 1 hp (with Toughness) then you should consider it valuable, because without it, you would be unconscious (and then maybe dead). The same applies if you've been to -9 (with Toughness), without it you would have been at -12.
The feat is designed to be taken by people with low HD, but it often pops up in requirements for "fighter" prcs. Fighter prcs should have a feat like Sturdy (or one based off of Fort) as a pre-req, since those feats are designed to help fighters. Toughness is always a great pick for a 1st level wizard, so it should also remain.
Technik