The OeBB KISS sets are to keep the semi-fast workings on Westbahn from delaying faster trains AIUI.
No, timekeeping on these slower RJ services is probably the best of ÖBB long distance traffic; they have enough reserves.
However, this line has frequent stops (on average every 15 minutes), is mostly used by people on shorter distances and RJ sets are not really adequate for them: capacity is too little, passenger exchange suboptimal, the restaurant not needed etc. So a very good decision was made to buy KISS instead.
In most countries, this would be a regional express or Interregio service.
I imagine OeBB would prefer to run RJ only.
Not sure about that (the Taurus is quite a track-basher at 230), and in any case, the RJ2 sets are quite different from RJ1. I except no further orders (even this one hung in the balance for very long).
IMO, ÖBB finally went for RJ2 because anything else would have taken far too long; the procurement would have needed to start at 0. And while KISS would make a lot of sense on much of this route, they cannot serve it before the Semmering tunnel is open. At least as importantly, I except some of these trains to run long distance into Germany (Vienna - Villach - Salzburg- Germany), and a KISS is not really acceptable for this.