coilovers? in the rear? on top of the leaf springs, surely you are talking about the front. replacing leafs with coilover and 3/4 link is a serious job.

I agree with Unfamilia and add my two cents.
With a mild increase in power axle tranp will come into play. An overrider leaf goes some way to helping with this issue, else traction bars such as caltrac will also do the trick (But only if required and after you have done everything else). I don't ever like the idea of flipping leafs it makes the leafs work against each other and adds the wrong arch/camber to the pack, but each to their own.

King springs do a leaf pack for the coupe (DAT-501) but these lower the car significantly and are pretty stiff. The concept is good, has on overrider leaf, but they are probably stiffer then required for a road car.
I have them in my coupe (CA18 - 330rwhp) and they help with axle tramp, but were very stiff on the road.

Remembering leaf spring add a lot of roll stiffness and generally negate the need for an anti-roll bar. Best/cheapest upgrade is to stiffen the bushings and tighten everything up first. Replace your rubber bushings with urethane (Upper/lower shacks front and rear), this will aid in suspension compliance (Pedders has most of them). Once you do this then see how it feels and if you still think you need an ARB then there are kits you can install.

Shocks, the best you can afford front and rear will make a huge difference to the ride and handling quality.

Keep it simple. Often the cheapest upgrades give you the best results.