Trans Mount Insert: I haven't heard any reviews of the Steeda one, but Travis at HSF had good things to say about the Fenfab one (same idea, not sure on durometer differences)
Rear Pinion Diff Bushing: I'd skip these. There are some horror stories about these. The diff can have some movement in it; it isn't meant to be solid-mounted. You already have a 4-bolt so just skip this one.
Subframe Bushing Inserts: 100% YES. Should be the first mod every owner of these cars does. Major improvement in rear end control.
FF Oil Pan: Yes. The data demonstrates this is an improvement.
FF Toe Links: Yes. The factory ones are crap that can bend when you breathe on them. I noticed a nice improvement (I have Steeda ones)
FF Vertical Links: Yes. Solid improvement. Just grease the crap out of cup contact points.
JXB Diff Inserts + Bolts: 100% YES. Major improvement. Should be done at the same time as the subframe bushing inserts.
PPE Trans Pan: I like it for the ease of future fluid changes. Skip the deep version; doesn't help like an older transmission.