Nope
All masthead rigged racing boats use backstay adjusters successfuly to control mast bend (and thus mainsail shape), and headstay sag (and thus headsail shape)-so do many cruisers.
It is true, however that those boats have bendy masts, and the 27 has a fairly stiff section, and in all of these cases there IS an element of compression-but the boat can handle a fair amount of this.
On the 27, the adjuster will provide a small amount of mast bend (quite small, but noticeable, right Nigel?), but mostly it will be VERY helpful in controlling headstay sag. You can make dramatic peformance gains by sagging it way off (loose BS) in light air, and tightening it up to limit or prevent excessive sag when the breeze come up.
VERY VERY worth it.
S