Some of the weirdness of the midi for this piece is because of an accidental 32nd note in the first beat of the first 6/8 measure. Instead, it should be a 16th note triplet like the rest of the notes in the measure. That results in a tiny missing time slice (the difference between a sixteenth note triplet and a 32nd note, which is a 64th note triplet) that completely throws TuxGuitar off when it tries to play the midi and makes it sound crazy. The tab is fine, since it doesn't specify durations.
I'd offer an updated version of the GuitarPro file, but I can't for the life of me figure out how to make the change in Tuxguitar. Graphical music editors are so quirky, you always have to learn them individually. I suspect the measure got messed up due to a bug in Tuxguitar, and it may not be fixable without deleting and re-adding the measure.
There was some interest in a piano version, I think I should be able to have one for anyone interested soon, I'm using Tuxguitar's Lilypond export function, and then cleaning up a bit, first thing on the list being replacing that 32nd note with a 16th, and grouping the triplets properly (which are also bugged in the Lilypond export).