Hey, I'll beat the dead horse a bit, just to add a little confirmation...
I'm also about 6'5", not quite 300lbs but pushing the limits of the 250's. I took the Poprad out on a test ride and it felt like a noodle under me. I was in a similar situation too I've been riding my mtn bike(s) exclusively for almost 10 years and accustomed to the feel of those stiff aluminium frames. The flex of the Poprad was too much for me which was a huge let down cause I liked everything else about it.
Just a note, the Specialized Tricross that I ended up with has felt rock solid since I got it, but there are not disc options on that frame.