Yes, alas I to have run into this problem on my hacked 1620. I've had a couple occasions where a disc would just not rip quickly. I've done some fairly extensive testing to try and determine the problem, over on the benQ forum... http://club.cdfreaks.com/showthread.php?p=939195#post939195
From what I can determine the problem is not with the firmware hack or a heat related issue, but just with this particular drive and certain discs. So yes it is slightly tempermental. But for me, the 3 discs I've run into out of about 50 rips really isn't that big of a deal given the speed the rest of the time. Although I must admit the slow discs ripped fine and fast on my Aopen 1648/AAP.
I hate to throw another name in the game but my old Lite-on 167t with the speed hack was a pretty good fast ripper as well.