Well I am level 20 and I have the 3 mods on IS and still I get more +% CA regen then -% to Aspect Pyro CA regen. I've made a little pictures here to illustrate my point.
First we have IS at level 1 and the regen time of my BT with level 1 IS activated:
Now let's see what happens when I get my IS to level 15 and the regen time of BT:
As you can see the regen time is actually more then with IS level 1. And I get some fire dmg on IS in return witch I don't find particularly useful.
The only thing I can imagine is the after some more levels (30+ or so) IS is actually going to give me lower regen( -xx.x%) on the Aspect Pyro then the
Regeneration Time of all CA (+xx.x%). But at witch point this is going to happen ? I don't see a table representing the values of the different levels of IS
in the Wiki(great Wiki BTW). You said you have 163 lv Pyro Elf so can you tell me when the +% is going to be less then the -% ? I hope I explained this well enough.