I would be astonished if there was anything in playing orchestral instruments making a difference. Both my DC got in the WGSB and WGGS on music and both played piano. My DS had just completed his grade 2 and played one of his grade pieces for the audition and got a mark of 42. I think my DD had only just started working towards her grade 4 so was really grade 3 standard and I do not recall her mark, not sure we ever had it, just relieved that she got in
Only you and their music teacher can judge which route to go but getting to the audition is actually quite a challenge as the music test has caught out many high grade DC with some not even getting an audition. If you look at my previous posts I have written at length on the music entry test. We did coach for it but it did not make a huge difference to their scores if I compare their first attempt vs later attempts after practicing. However it does help with confidence knowing what to expect.
I should read all DAOGroupie posts first as I have repeated what she said