I can't post the link to this, since it's on a restricted access website (but any SETP members will find it without trouble), but...
Quotes from a paper called:-
SHUTTLE CARRIER AIRCRAFT FLIGHT TESTS
by FITZHUGH L. FULTON, JR. (F)
NASA, DRYDEN FLIGHT RESEARCH CENTER
Published by Society of Experimental Test Pilots, 1977
Takeoffs
Takeoffs were conducted at gross weights from 554,000 pounds to 626,000 pounds. The simulator-developed technique of gradually increasing thrust to takeoff power by the time the airplane reached a speed of 60 KCAS was confirmed as a desirable standard technique. However, it was found that a more rapid thrust increase (with takeoff power set by 30 KCAS) could be tolerated. Under these circumstances, nose strut extension
was enough to cause a momentary loss of nose wheel steering. However, the steering loss was of short duration and caused no problems. The rapid thrust increase technique would be useful for reducing ground roll when taking off on a short runway. Rotating to a pitch attitude of approximately 7 degrees for liftoff gave a tip fin clearance of about I O feet. The rotation technique causod the takeoff speed t o be 5 or 6 KCAS higher than for an
unmodified aircraft and added slightly to the takeoff distance, but it was satisfactory for this special purpose airplane. The ground roll distance at a gross weight of 554,000 pounds was about 5000 feet; this represented the launch mission takeoff weight.
MATED SCA TESTS IN FERRY CONFIGURATION
When the ALT program is completed, the SCA is to be used to ferry orbiters wherever they are needed. The tail cone will be on; the primary difference between the previously tested mated inert configuration and the ferry configuration will be a change in the angle of incidence of the orbiter on the SCA. The forward attachment pylon will be shortened to give a 3-degree angle of incidence, which will reduce the drag due to the orbiter
considerably.
Three flights with takeoffs a t gross weights up t o 710,000 pounds are planned to recheck flutter clearances, obtain stability and control data, confirm configuration airworthiness, and compile the necessary performance data for the ferry missions.
CONCLUSIONS
The modified Boeing 747 has proven to be a satisfactory carrier for the orbiter. With the orbiter attached, the shuttle carrier aircraft’s handling qualities, performance, and stability and control were good or better through-out the envelope tested. And finally, the launches of the orbiter with the tail cone on have been satisfactorily completed.
All a bit vague I'm afraid, and a distinct lack of hard numbers comparing the performance between the two - but I'd read between the lines and say that implies that the lift available was significantly increased.
G