Maybe a question especially to Stelios, as he recommends it quite often - but likely more people here are using it, and can give some info there.
I'm currently using the TSFLAT2 from Teleskop-Service for my Astro-Physics Stowaway (old).
While it does a good job (I think) in flattening the field, it does introduce chromatism, I do have to adjust focus when changing filters. This is a bit nasty, because I use an OAG, so my guide star gets defocused when doing that
So I started looking at alternatives, most notably the Hotech one (I don't want a reducer, only a flattener). So the main question would be whether you have such issues with the Hotech when using it on an initially color-clean APO refractor.
The other question came up while looking at its specs: It has a 'self-centering' adapter that compresses rubber rings to clamp itself into the tube of the focuser. Is that indeed stable enough to support a full image gear (EFW, OAG, ASI1600 main cam and ASI290MMmini guide cam)?