How good can you get on an upright piano?

[etogmajor]

I don't technically have an upright. I have a yamaha NU1 piano - that is, a digital piano with literally the same mechanics of an upright minus the strings. I'm trying to get my ARCT diploma. I got 88 on my grade 10 exam recently (I'm talking Royal Conservatory of Music in Canada). I'm having a great deal of trouble getting anything up to speed.

Example: I'm playing Beethoven's Sonata op. 81a "Les Adieux" and at the moment learning the 3rd movement. I hear most recordings at around 95-100 to the dotted quarter note, but when I approach 95 I find I'm constantly inaccurate suddenly and it isn't going away. Some of the rapid scale passages are simply too fast. I don't know what I should blame this on, but I've played on a Mason and Hamlin 5'4 grand piano and the action was so much lighter and quicker than anything I've played on since. I feel like I could more easily approach that kind of speed on that action or similar.

Also, any speed I get on my piano is instantly gone the moment I switch to my teacher's studio grand Heintzman (the Canadian type) which has the least tight action of any piano ever because it's so old.

My main question is this - should I not try to approach speeds I hear on recordings because my piano simply won't do it? Should I give up on my ARCT because there's no way in H-E double hockey sticks I'm getting a grand or access to a grand?