I would definitely recommend trying this BCD on, or any BCD for that matter before final purchase.
I bought one of these last year and can't say anything bad about it. It does what it says it does and you can't really fault that.
With it being a back inflating BCD, the sizing becomes even more important because when the BCD is inflated you don't get that squeeze you usually get from a normal jacket BCD. This can usually off set a small degree of over sizing. With the Pegasus, if the sizing is too much on the large side it will tend "suspend" you like a air balloon's basket! The straps offer a lot of flexibility, so I'd almost say that you could go 1 size down and still be perfect!