To have run at a stable speed that is under some form of speed regulation, the motor control board would have to be altered or a larger flywheel which drives the capstan would have to be installed and I don't know if there is enough room inside to accommodate that?
After the mechanical issues are addressed, the calibration levels and eq would have to be completely reworked to allow the response to remain reasonably flat and so as not to throw off the dbx encode/decode process.
Some bottom end response may also be lost due to the head gap designs for it's designed speed of 3.75 ips.
As well, with regard to the heads, they will also wear down twice as fast at the faster running speed.
All in all, it's a big undertaking to perform with questionable improvements.
Cheers!
Of course, it makes perfect sense that if you change one thing then all others must be "tweaked" as well. Thanks for explaining Jeff!