27/15, 21/21, 15/27 all seem pretty close to me.
27/15 you'll always feel that you want to be a little faster
21/21 You'll be fine most of the time, but feel the differences when you fight a strength build or an agi build and have to change under both conditions.
15/27 you'll always feel a little weaker and are punished more for mistakes.
I think HP vs. speed balances each other out as far as surviving is concerned. Internally, I think the HP gained from strength overpowers the bonus from IF, making IF a little weaker than it should be. Unfortunately, it's not much that can be changed. I think giving certain armours IF requirements sounds like an innovative idea.