On the top speed issue Jon is correct. The Sundstrand manual specs show that at full forward the pump displacement equals motor displacement and you have a 1:1 ratio input to output. The output then goes into a 7:1 reduction gear which then goes into a 3.54 ring pinion gearset. So running through the numbers at 3600 input rpm we end up with an axle speed of 145 rpm. With a stock tire size at 23 inches that works out to 873 fpm or 9.92 mph. Without mods to increase pump output or some gearing changes that's what it is.
Now on the fast acceleration, if you are a 100% sure the external linkages are good then the only thing I can think of is the coil pins connecting the trunnion input shaft to the trunnion are loose letting the swashplate move farther than what the control is moving (have never seen this happen but anything is possible) . Hydros want to go full tilt naturally and the controls actually hold them back. Check the trunnion shaft at both ends to be sure they are pinned securely.
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