One can never be certain, but I would be very surprised if Frolik and Setoguchi aren't a significant upgrade. Frolik is a two-time 20 goal scorer, and a fantastic penalty killer. I think that Setoguchi gives us more pop on line 2/3 and will improve the power play.
I think that the other potential improvement from last year could be health of key players. We started without Bogosian for the the beginning of last year and played most of the season without our top LHD (Enstrom). The Jets still aren't that deep, but if their top players stay healthy I think they'll be better.
Beyond that, I expect continued development and improvement from Kane and Bogosian; two of our key players.
The wild card might be Pavs. If he nudges his save % up a bit (maybe by playing him more judiciously), it could make a difference.
Historically speaking for results (not style) with compairing to Frolik and Setoguchi:
Wellwood is better at outchancing opponents and points than both at 5v5, although the other two are more goal scorers while Welly is a playmaker. Frolik is the better PK player and Seto is the better PP player.
Antropov is a better 5v5 at points than both (again playmaker vs goal scorer); however, Frolik is better at outchancing opponents. Frolik and Antro are similar in PK efficiency but Frolik fits the Jets system better. Setoguchi is a better PP performer but it's close.
Burmistrov and Frolik have near identical point production, usage, and out-chancing for 5v5. Burmistrov has historically been a top 20 PK player in the NHL over the last 2 seasons; Frolik is good but not at that level. Seto destroys Burmi at PP.
Miettinen is terrible and made everyone he's with worse... except maybe Thorburn but he has the nose.
But there's a bit of apple/oranges going on.
It comes similar to the argument Jeff Veillette had about Bozak/Clarkson versus Grabovski.
Grabovski given the same treatment as the other two would perform better; however, that is unlikely going to happen due to coaching and system.
Wellwood and Antropov don't really fit the Jets direction in system, so there is a possibility of the others being improvement in overall effects due to those factors that if Welly/Antro stayed they would not likely been optimized.
TL;DR version
Optimized usage Welly+Antro > Seto+Frolik, but in WPG system Welly+Antro ? Seto+Frolik