Other more knowledgeable souls may disagree but I would not expect much of a change after servicing with a watch running with a modified Sellita SW200 (I'm pretty sure thats what IWC use as the basis of the movement). A standard sw200 can be easily regulated to run close to 0 secs/day - does IWC state an acceptable range for the rate variation on this watch/movement? That would be something I would check first and if -10 secs is acceptable to IWC you may be in for a struggle to get them to take it back and adjust it but I would still push for that. I once bought a used AP Royal Oak (under gurantee still) that had pretty lousy time keeping but the AP blurb stated a wide range so I was snookered with that and got rid of the thing the following week.
Good luck
Keith