The progressive throttle sucks, do the throttle cams really work? Best one?
The throttle cams do help feel but not performance. Automotive sells a good one.
With out hearing / seeing your car run it hard to tell. I lost a timing belt recently in my 924s and it started out just like as you describe. Check compression, check vacuum and triple check that the timing is properly aligned between the cam and the crank.
Chips. Yes but will at bet get you 8-12 hp and 500 more rpm. They will help the low end a little. I liked the weltister chips.The wilks ones also have a good rep. I got much better results by installing a programmable EDIS based ignition system.
Shifter:
The stub connection where the shift rod meets the strict is probably worn, bushings back at the trans are worn. Cure is to replace the shifter or if you can weld a 10 mm bolt welded in place of the stock stub on the shifter fixes things. the bushings in the back are replaceable. Short shift kits are installed back at the trans. Only way to get other shift knobs to fit is cut the handel and weld on a bolt with the proper diameter for the knob you want. I did that and it worked very well.
Do the timing belts and WP, There really is not alot more HP to get out of NA 944's Want more go get a 951
Improvements that do help are: Koni Yellow sports all around are the best mod you can make. Followed by weltmeister sway bars, Iboc front springs and weltmeister rear torsion bars. The torsion bars are a PITA to install and provide the least improvement of the things in that list.
Brakes really do not need improvement just go get really good pads (porterfield or coolcarbon are a good place to start)
As for tires. I drive on the street with 195, 65 16, I have 205, 65 16 Yokes and 225 55 16 R1's. The 225's are faster but will also get you in deep you know what much faster. They are not at all forgiving. But when driven correctly they are very fast. The 205's are much more forgiving. You will feel the limit coming. They give you a chance to collect things. Yes they are a tic off the 225's
Respectfully, Dean