If you're going to run over 6psi, chances are the OEM clutch will start to slip. A clutch rated at 200 ft lbs. of torque should be able to hold to around 9-12psi. A clutch rated at 260 ft lbs. of torque should hold against everything you throw at it, within reason (about up to 16 psi on a 1.6L). There are a number of manufacturers that sell clutch kits.
Typically a kit includes the Pressure Plate, Clutch Disc, Throw-out Bearing and Pilot Bearing. You’ll want to replace all of these when upgrading your clutch. Surfacing/replacing/light-weighting your flywheel would also be beneficial at this time while it can be removed. 1.6L Miatas can upgrade to the 1.8L clutch when coupled with a 1.8L flywheel.
I (cardriverx) have had the F1 racing stage 3 metallic 6 puck clutch for about 14k miles now. It works perfect for me, it just grabs a little harder than a typical organic disc when you take off. It holds my 230 HP fine. Overall I feel the Ebay clutches are of deicent quality, and problaly worth the money. The throw out bearing feels a little flimsy compared to O.E units, but so far so good for me…
Of course the metallic disk will be harsher when launching the car and doing hard shifts, so if you try to chirp the tires going into 3rd gear with 300 horsepower, don't be surprised if the transmission explodes!
Most people tend to go for the ACT HD clutch (HD pressure plate with organic disc). It is pretty much the best route to go, but expect to pay at least $500.