I have this game installed onto USB, below are some quick tests:
- Launching the game from the WiiU Menu, it took around 1m:03s to reach the title screen.
- Created a new game and it took around 1m:05s to get into the game.
Did the above test a few times and the times taken each time are pretty close
If your having long loading times from the retail disc, looks like installing to USB doesn't improve for this game, for long term, it would be a good idea to try and install any retail disc games you have to USB, this will preserve and not wear out the laser in the drive.
I'm using a 2TB USB3.0 Toshiba USB HDD with Y cable, with 645GB free space, I have EVERY single EUR/PAL retail game installed.