Slitherine

As simple as the thread's title.

I got my artillery group ambushed by enemy planes and my fighters will take a good 2 turns to reach them.
I would like to avoid reloading previous saves, but the elite-reinforce cost is astronomic.

So, is it worth it? Will they lose suppression/entrenchment damage and raw damage?

I only use it for Infantry because the difference is not huge in terms of cost.

I always elite reinforce, except for 5 stars which can easily recover the lost star.
However artillery is outrageously expensive.

No, absolutely not. Arties have not so much different being veteran. The most benefit from high experienced units is getting tougher, and with arties, its definitely not so important if you play carefully enough.
For me, 3-3.5 stars for infs and 4-4.5 stars for tanks/fighters seem most suitable. Except for end-camp when you will swim in Rps and want to keep most experience to next camp.