Hi Dave, I have had an HW98 for a couple years. In stock form they do have a bit of spring twang to them. I have had mine set up with both the Vortek FAC kit and now the Vortek 12 fpe kit. In answer to your questions;
Scope Mounts - I am using FX No-Limits. I have learned the importance of setting my springers up with adjustable mounts to allow me to set scopes up at optical center. Much less chance for POI shifting from shot to shot. Keep in mind, I am now setting all my springers up in the 12 fpe range. I have had no problem with them moving on my 12 fe rifles. I can't answer to how they will hold up on FAC springers. I also use them on my PCP rifles, excellent mounts.
Accuracy - The HW98 is an exceptionally accurate springer. You'll hear folks arguing that fixed barrel undrlevers are the most accurate springers out there. I'm not disagreeing with this, but the HW98 gives them a run fo the money! At 50 yards, 1/2" center to center groups are fairly routine with the HW98, with my smallest 50 yard 5-shot group being 1/4" center to center. If you look in the Springer groups post in thie forum, I just posted a couple pictures of 50 yard groups shot with my 98, the most recent being just this moring.
Preferred pellet - I have always found my 98 to prefer the AA 4.51's
Other suggestions - If you are a shooter of larger stature, you might find the stock a bit short. I am 6' 1" and added a polished aluminum spacer to my stock. This gives it a much better fit for me. Trigger pull form the factory left a lot to be desired. I like my target rifles to be set anywhere from 10oz to just over 1 lb. I honed the sears and relubed, then set to a shorter pull and lower weight. The adjustable cheek piece is a really nice feature, and the light weight of the rifle makes it a nice offhand shooter as well.
Hope this helps!!
Steve