Wally put together a list of things to look at regarding up shift points for a lister's '89 S4 that seems to be shifting early. There's a very basic, maybe too basic, thing to consider, and that is checking to see if full throttle pedal equals full throttle open. More than a few cars I've owned have had maladjusted linkage, stretched throttle cables (very popular on carbureted 911 models), or just had things bent up at some point by abuse or lack of attention during maintenance.
The simple test: Have somebody mash the pedal to the floor while you watch
the linkage under the hood. With the pedal at full depressed position,
manipulate the transfer linkage to see if there's more movement available.
My S4 has a spring in the 'pull' cable from the pedal that allows full linkage movement just prior to full pedal travel. There should be a slight bit of compression in that spring when the pedal is at full depressed position.
Having a throttle that doesn't open all the way robs you of some available
performance. Having the linkage travel limited by something bent or not
adjusted correctly also means that up shifting shifting will be early.
This is a simple thing, too often overlooked. Causes besides the mechanical ones listed above include carpet and mats that are bunched under the pedal, loose golf balls and pop bottle lids, even mud and gravel scraped off your shoes accidentally. A little cleaning and lubrication of the pedal linkages in the footwell is a good idea anyway. FWIW, the 911 cars are the worst as
far as fouling the pedals, with the linkage pivot at the floor. The 928 doesn't get too much gravel flying up into the linkage.