In the Netherlands 98 octane is getting rare, but my car is mapped at 210 bhp on 98 octane. What kind of loss can I expect when the car is running 95 octane? Anyone ever measured? I know the engine will adjust to this fuel, but I also understand they need 98 in OEM map to run fine 

It will run as normal, but you may experience knocking, depending on how aggressive the map is. As far as I've researche online though, 10-15% loss maximum, but it all depends on the map!