d20 - Alternate HP System
Your system makes HP much more plentiful at first level, and as you put it... it makes their advancing a little slower in many cases. The fact that it makes them plentiful will make even a HP advancement that is closer to before still 'seem' proportionally less significant. That might make more of an impact than the actual technical aspect of the change.
Certainly, certain attacks are scaled 1 die per level or two, and so while it might seem like very little to begin with, the advancement as it goes on may actually start feeling more significant. (that isn't necessarily a bad thing, but it is worth notice)
Honestly, when I first had read your % advancement, I too had read it % of previous HP which, I thought was not simplifying the math. With it only being percent of the base, it makes much more sense as a simpler calculation.
I do want to mention that while I'm fine with the multiples of 5 for your bonuses, I have to mention since you are including a person's CON score which need not be an even or round (as in multiple of 5) number, adding multiples of 5 doesn't really seem to simplify things significantly. With that in mind, if you are starting over with classes etc. Go ahead and pick what bonuses you want, but my suggestion is that if doing a straight conversion... use the MAX HP as your bonus to your CON base.
So my suggestion, a Fighter with a CON of 15 would have a base of 25(15+10). While a Barbarian with a CON of 12 would get a base of 24(12+12), and a Wizard with a CON of 8 would have a base of 14(8+6)
Honestly... It brings up the question if it would be simpler to simply swap the +CON Modifier per HD with staring out at first level with your CON in HP and perhaps simply max your HP per Die as you advance.
My biggest gripe with 1st level is how getting to second level you are seemingly twice as tough. That seems like like a big jump, while latter levels it is less noticeable. Admittedly, in reality, at first level, since you normally start with max, unless you roll well for your second level, you normally aren't actually twice as tough... but it still gives you that 'feeling'.
Doubling after five levels seems a reasonable advancement, so if you wanted to use 20% it wouldn't seem too much (you might even get away with 25%).
I have to also admit that I have wondered if Size shouldn't play more of a roll in creatures HP more directly. As an example... why does an ogre have to start out a higher level. A part of me would suggest have the creatures size potentially adjust their base. Be it a modifier against the whole base, or as a modifier against the CON, etc.
Tiny = 1/3 Con, instead of CON
Small = 1/2 Con, instead of CON
Large = 2x Con, instead of CON
Huge = 3x Con, instead of CON
[or maybe it would apply against the whole base? Granted, that would be a significant change, might have to look at how it impacts various creatures as you did before.]
Also, are you considering what the influx of HP will do to your natural healing rates?