I'm personally ok with heal as it is, but here are my ideas if it were to be changed, but I'm not 100% onboard with the diminishing returns...
what if it were something more along the lines of a medkit when used? Like instead of diminishing returns, it gives you a flat 80% hp back. And if you are BW, maybe remove the BW and give 40% hp back?
The other idea I can think of is basically making the full heal cost more, but including a smaller version...the !heal is 20 points and is 100% and the !heal2 is 10 points and 50%? Call them !largeheal and !smallheal?