Being an original Bushmaster Deluxe owner, I must say, your's is in amazing condition, looks like original anno, no visible pump rub marks, all original screws and, no serious scratches.
That being said, what are the internals? From what I can see it looks like you may have the stock bolt (vice a nelspot compatible). What kind of Hammer and Powertube do you have? The stock barrels are massive overbore for .683 paint so, that alone would explain difficulty in getting velocity up.
FYI- your's is a pre-90 deluxe model (in 90, they redesigned the rear of the body and the ASA so that you only need one screw vice the current three).
If you play with this marker, be advised that the various screws have a tendency to work themselves loose and a lot of folks lost them when they fell off on the field. I still have all mine (aside from the PITA Block Screw (aka barrel retention screw) that broke under warranty) and I think it's because I bought a bunch of tiny o-rings to slip over the screws like washers (for friction).
I know a couple different folks here had their barrels freak bored but, I couldn't address just what their solutions were as I agree, the retention ring/groove does bring the O.D. pretty close to the I.D. You may want to get another barrel to try and Freak bore as your gun is in such great cosmetic shape (matching anno and all).