I see that whiteline have an 18mm and 20mm option for the rear and a 20mm for the front. Is it better to go for the 20mm rear bar on an MR2 due to the extra weight from the engine? I also see on the  (two's r us) site that they say the reinforcement plates current design only fits cars manufactured before 10/91 has anyone been able to fit them to a 94 model car?
Also has anyone found any major differences between the RM series and the BR series? I will be driving the car regularly on the street with the hope to be able to still do the odd track day.

The first thing with suspension mods should be to be sure ball joints, rod ends, shocks, and control arm bushes are in top condition. Only then worry about other stuff.
 
'94+ SW20's usually have 18 mm ARB's fitted as stock both front and rear.
The 20 mm Whiteline adjustable front bar is a popular mod for SW20's to sharpen up turn in.
But for a car that is used mostly for the street, consensus seems to be that a heavier rear bar is unnecessary, especially if you are using stock alignment settings. 
 
 

RM series users inverted shocks to reduce unsprung weight. Good for racing but not so much for street. My rm coilovers are leaking grease and rattle quite badly at the front. When they are completely shot i'll replace the front shocks with br units. For a street or street/track car I would go for the br.

Adrian
My rm coilovers are leaking grease and rattle quite badly at the front.

Check the strut top nuts, I've seen a few cars with rattly coil overs that it turned out to be this nut having worked loose a turn or two allowing it to float on bumps.

Thanks mate. They've come loose before but they're tight this time. Tightened every bolt and collar. It's the shock itself. Because it's inverted, it sits inside another sleeve that then screws down into the base. Between the shock and the sleeve there is a thick red grease. The seals have broken and the grease leaks so I think that's where it's rattling because there isn't enough grease left.

Ah, in that case, probably worth dropping them into a shop to get a quick rebuild on the seals and regrease. I believe I have read elsewhere on the interwebs that the RM's are fairly rebuildable since they use a very common design.

Adrian how long ago did you buy these? Wasn't it just earlier this year? They need to be rebuilt already?

Had them for more than a year now. The damper is ok I think. Just the grease packing and seals it sits in. Pretty annoying but still works for now. Will need to be re greased and seals fixed or just thrown in the bin later on and replace them with regular be dampers that should last longer. The rears are fine. Just the front playing up.

A bit of thread revival..
 
Just curious if anyone here knows if you can get custom spring rates for the BC coilovers or if anyone has changed their spring rates and if so what were the new spring rates you went with.
I am starting to think I would like to go a slightly softer spring rate than the BC factory set F5kg R8kg as for me they feel a bit too stiff when on a bit rougher road. For comparison our 2002 STi is also running coilovers but with what seems a softer spring rate to what the BC's in the MR2 have and the STi has completely different behavior over the same rougher road, much better to live with and has less tendency to want to skip over corrugations or ruts on the road.

How's the dampening setup on yours? The fronts are just 1KG stiffer than Tein SuperStreets which are the softest (most comfy) on the market.