Glancing at your specs, I'd definitely make it a 9-string. Your tuning scheme is perfect for that - just add your high E and you've got 3 sets of major 3rds spanning 3 octaves. I'd very much recommend using a multi-scale (fanned-fret) design - you could keep a 27" scale for the low Ab and use a regular 25.5" for the high E. This is a very subtle fan, and I'd bet that you'd find it more comfortable than straight frets, especially if you've got smaller hands.
I've got a 9-string 28.5" Ergo guitar and the neck is extremely comfortable. I always tune it strictly in 4ths, but between you & ElRay talking about major 3rds I think I'll give that tuning a try.
Good luck with your project, and keep us informed.
