That could work. One of my grade 8 pieces for trombone was a 3-part cello suit adapted for trombone. Itssemi-quaver galore at 120 BPM.
And yes speed/fitting in as many notes as possible is not the end all be all. I also had a piece in 3/2, very slow... Gustav Mahler's 3rd Symphony trombone solo.
Its all about you really. ABRSM/Trinity GuildHall always seem to have a kindof snobish attitude to modern instruments such as guitars, etc.