I think some people have done that (maybe with HyperPro springs).
The problem is the valving in the shock is marginal even with the stock spring, probably going to be worse with a spring rate it wasn't designed for...
I'd personally (and this is my plan) just adjust the spring to the max (190mm) setting, live with it for a while, and save up for a Nitron R1 or K-tech.
The aftermarket shock will be much better, valved and sprung exactly for your weight (at least the Nitron - not sure how K-tech specifies spring rates).
I think even if you put a stiffer spring on now, you'd eventually want to upgrade further and you'd then be $100+ further behind.