Yes, I agree about the hp bars. They are designed in a way that you can't see the ends of them anyway in the client, so they do already give a bit of a skewed perception of how much health you have. Adding some ironflesh is the tweak I think would be least likely to cause any major glitches.
When it comes to powerstrike, it depends if that stat affects how fast you swing. Weapon skill does affect swing speed right? And power strike only adds some umph to it? If that's the case then power strike should be fine to tweak. I haven't seen the actual source code for this though so I'm not sure, but this should be quite easy to test.
As far as testing goes, my concern is that tweaking a stat a point or two might not cause any obvious bugs that you notice right away, but could increase the frequency of weird stuff like instant swings, small animation stutters, ghost range, horse lag etc. It would take some careful testing to make sure this isn't the case as even a small increase in these kind of glitches would have a negative impact on the gameplay experience in the long run.