I forgot the add on your earlier
THREAD, you won't actually notice any improvement in braking by fitting 105/Ultegra calipers.
They're basically the same caliper but each one is just fractionally lighter than the one below it, (it might not be quite as simplistic as that TBH but it's not far off).
The best brake upgrade you can make is in the pads. Use either Swissstop Green or Koolstop Salmon pads for improved performance. Then if your still not happy you can still upgrade your calipers and swap your upgraded pads over anyway.
PS: Shimano brake pads are made of cheese
