you dont need a ZS rear beam to stiffen the back up, just know where to weld some plates on the rear axle
although saying that my rs handles fines, ive had the back end slide once and that was at 20 round a bend when it was real frosty, but ive got 15's and decent tyres.
i also run on avo coil overs, never had a problem with them, but then i never really had a problem with standard apart from more body roll.
ill be going to 16 inch wheels soon so will see how it handles then but i wont race my car about like i used to, not after the time and money ive put in to it over the last year.

have to say though, my B reg xr2 handled a bit better and that only hand lowering springs , not £550 worth of coil overs

your be pushing it for 250 bhp on a cvh, it can be done but why not just build a ZT or ZVH?