I'll throw this out there as something to check although I don't think this is your issue as your blow out isn't as bad as my one helper had. Have you lifted the fron of the mower and rotated the blades and confirmed you paddle in the vacuum is rotating the right way? My helper put the bagger on at one account and had the belt routed correctly around the pullies however he had the V side of the belt running around one pulley rather than the flat side. I have a scag bag system, not the one you have. Anyway, this caused the paddle to run backwards. Basically what was happening was the blades and paddle were fighting each other and I was getting major blowout. Even though it was running backwards it was still filling the bags (just at a slower pace).
Like I said I doubt this is your issue because your blow out is much less than what we were experiencing, but figured it could be a quick check for you.
Best of luck to you. I know how frustrated we were until we figured this out.