I've seen a couple of good grows now with Heavy Harvest, but I haven't used it myself. I'm not sure that it's all that expensive, given how big of a harvest I've seen people get. Obviously, it depends on how many plants you're growing and your growing conditions too, so I'm not sure any company could guarantee certain results.
All that said, I've had a TON of experience with Advanced Nutrients other nutes, and I think if you're going to invest in any nutes, they're the best of what I've used.
Outdoor growing is a bit tough, no matter what you do, but I think going with a company like AN that works to help MJ growers specifically is going to be your best bet.
I say go for it.