It may be a daft question, but have you checked 100% that your spray is working properly? If you've got set off in one area, then it's possible that you've got a blockage somewhere or the pattern is not set up properly. I know that happened to me years ago and it was a beggar to clear the blockage, I think we ended up buying a new part.
We have one press without IR and one with (+coating) and the IR definately speeds up drying and reduces spray a lot. Having said that, we (almost) never have set off on the press without IR and if we do it's always because we've bolluxed something up (technical term there).
IR are also fairly expensive to retrofit, I got a price earlier this year to fit one on a newish Lithrone 28 which was already factory "prepped" for IR and that was GBP40,000. So, much cheaper to sort your spray / working practices out IMO.
We use IR & coating to be able to W&T immediately and so that work can go immediately to the bindery, not to fix set off.