Forget the inferno set, it's way too oversteery for a beginner on the XRT especially a mouser. It doesn;t help your using the mouse because its hard to get on the throttle smoothly, you sorta have to tap it a few times before you can mash it down.
I've attached a replay of me driving using the mouse and the set I used when I was a demo racer using the mouse. I don't remember my PB but I just did a few laps and got a low 1.27 easily, so 1.25 shouldn't be a problem. It's a very stable and easy to drive set, and also doesn't lock up when you brake with the mouse.
Check the replay and you'll see what I mean by the throttle control required, you should shave a couple of seconds off in no time.