Filing brake mounting holes...?

I've fitted a set of Rx4 calipers to my Planet X Tempest, and noticed the flat mount holes at the rear don't quite allow for central positioning of the caliper (ie the "slot" isn't wide enough).

Was wondering if it was worth attempting to file it a bit to allow more accurate positioning. At the moment it's causing the rotor to flex a bit but it's not terrible...and I'm not sure how easy filing titanium is!

Anyone done similar? Did they die?