Even on 8 man raids its still fine because force armor eats up quite a bit of damage giving you more than enough time to top someone off. Plus when healing trance is criting for more than 2300 its more than enough to single target heal a tank.
If Surge made a linear contribution to crit multiplier, that would probably be a reasonable approach.
However, despite the fact you "don't care" about diminishing returns, the difference between 300 and 600 surge rating is going to be around 40 - 45 more HP on crit ticks (not a cent on non-crits, obviously). This versus the entire channel completing 9% faster? Not even close.