The green line is the STF-2 in my room (one step up from the STF-1). I'd say HSU's advertised measurements are pretty spot on. It also shows you can't always rely on room gain. That said, I'm going to echo advice I should've listened to when I bought my first sub - Go for as much performance as you can afford. No one ever regrets overkill.
The red and blue lines are my DIY sonosub. The difference between it and the STF-2 is substantial. It also ended up being a good deal more costly.