They are not classified strictly as a semi slick tyre R compound however Federal sell and market them as a track oriented tyre. They even refer to them as being a semi slick in there marketing and website. The treadware rating on them is 180 so they are much softer then a  high performance street tyre but still much harder then a soft R compound. 

I only use the car for racing so after proper semis, I have a spare set of wheels/tyres for rainy days

I think you should look at the Yokohama A50. The Nitto NT01 tyre I was running was ok but do have their limitations.

Shane's run the RE555, the r888 and whatever the Hankook z221 before on the Gorilla. Also we've tested the Federal FZ201 on a skyline.
 
RE555 were the best performance but not worth the premium price. The hankook would be the best bang for buck.
 

Dylmrt, what kind of lap time difference were you getting between the 3 tires you tried on your car? What size tyres were you running?

Not on the SW20. this is on shane's supra running 295s. Hard to say the time differences purely attributable to the tyres given the car is constantly changing.

Here is a link to a good track day comparison of street vs track tyre using MR2 Turbo's.
 
http://www.tyrereviews.co.uk/Article/Road-VS-Trackday-Tyres-Mega-Test.htm
 

I currently have RE555 on the front and A048 on the rear but looking for mid range budget tyres. The RE555 are great.

I guess there are three important factors that need to be thrown in. What size tryres do you need, what price did you want to pay and how long do you need them to last? I take it you mostly run in club days where you get about 10 laps per session? I've only used the Fedrals so I cant give you a comparison to what they are like against the RE or Yoko's but they are very cheap which is the biggest driving factor for me and I'm happy with how they perform on the track. I'd buy another set again when they wear out. I probably have around 300 engine horespower or so and use 255's on the rear. Federals cost me around $700 fitted for all 4. So half the Price of toyo 888.

By the sounds of it, Pat wants proper semis (RE555/r888/Z221/FZ201), not the "street semi" class (RSR/AD08/StarSpec/KU36 etc).
 
Whilst I love the Federal RSR for bang for buck, in proper track racing, it doesn't stack up against the proper semis. IT doesn't even stack up against the Advan AD08 (1 sec difference around Eastern Creek).
 
I'd give the hankook a go Pat. Shane did his best times around Wakefield with them.