Like everyone said, if your brakes are locking up, then the juice is there.
Are you using mostly your rear brake? Even with a 8 inch rear hydraulic setup, if a rider just uses his/her rear brake only, you're not going to stop very well. Front brake is key. The physics of a moving vehicle will always need to rely on a front brake for the majority of it's stopping power.
I come from a BMX background, where I only had a rear brake. I brought that habit with me to mtn biking and I had so many sketchy descents and I just couldn't stop well with my habitual rear brake technique. Then I started to learn how to properly use my front and it's been SO much better since.
If it turns out that you are using mostly your rear, start practicing with the front. Some sprints and stop with just the front, gradually adding more power with each sprint/stop as you get comfortable with the threshold of it's power and learn to detect the point where it will nose wheelie the bike.
185 pounds is not too much for a 6" rotor to handle. A parking lot brake test will always seem awesome because you are not going downhill where the brake is truely challenged. If you observation that your buddy's Hayes were stronger based on a parking lot test, I'd say that's not an absolute indicator of brakes. Some brakes are more grabby than others in low speed parking lot test.