I would approach this one of two ways:
1. Do it all, oil change $30, new springs $8, new plates $150, check cable adjuster if cable is has adjustment left or not.
2. Try oil change $30 and new springs $8 only. On some of the bigger vtwins an oil change helps 4th to 5th gear shifts for some unexplainable reason.
The problem with number 2 above is if it doesn't solve the problem you're taking the exhaust back off, foot control, and side cover to get back in there to put in new plates. I guess it comes down to how important is $150 to you if you'd just do it all in the first place.
1998 Shadow 750 ACE
Bunch of others