calling 355 experts

I have a '98 GTS that I imported from Italy in '00. The car has a TUBI. 20k miles on the odo. I have had the major- engine-out, belt and clutch replacement service done 3 years (and 5k miles) ago. The car is driven moderately hard and put through it's paces 1x/week, pretty religiously. It runs pretty damn strong, but I have only driven another '05-B to compare it to, and that was 5 years ago. I do not seem to have the exhaust manifold or valve-guide problems <yet>.

My questions...
1.)Norbert Hofer (GT Classics)is my mechanic. He has recommended that we clean and replace the fuel injectors due to the car idling at too low RPM(950-1000). Another symptom is occasional "boggy" acceleration at lower rpms.
GOOD IDEA?
OTHER POSSIBLE/LIKELY CAUSES??

2.)What are the top "bang-for-the-buck" things that y'all think I can/should do to make sure the car is running optimally and producing max power.
-DO VALVE GUIDES AT NEXT MAJOR?
-AIR BOX MODS?
-HYPER FLOW CATS & AFTERMARKET HEADERS (together like $6-7k installed, right?)
-ANYTHING ELSE??

3.)I am having a lot of vibration and noise from the shift lever in 3rd gear (primarily) and occasionally 4th. This always happens when cold (I skip 2nd) and diminishes as the car warms. 3rd still doesn't seem quite "tight" and noise-free when fullly warm though. It seems to me that something is loose or worn, but I don't want to spend 3 shop hrs. of labor trying to diagnose that a cheap bushing (or other small part) needs to be replaced.
WHAT TO DO?

Thanks for reading and sharing your experience and expertise.

 

greyboxer, go read your response again. It immediately casts dispersions on my intent and relationship with my mechanic-both of which you know nothing about. Then you tell me to just figure it out for myself. As for sticky linkage or valve suction(which is a very vague description)...your response is less than enlightening.
Perhaps you're trying to help-and you're a legit. owner/poster on here- but I read your response as trying to be antagonistic. Please clarify.
-Rob