I'm thinking that it is time for a new set of leads and plugs in the old girl. A bit af a miss at idle and a bit unwilling to rev!

My question is, are the thick (you know 9.5-10mm) really high quality leads and the NGK iridiums really necessary on a bog stock EB 5.0?

What gap and heat range would be best suited to this motor (run entirely on premium, usually 95, sometimes 98)?

Is it worthwhile replacing the coil at the same time, considering I think it is the original one. Do they become less effective over a period of time or just die one day?

silicon leads are have lower resistance so more voltage to the plug for a bigger stronger spark (mind you we're not talking massive difference here).
irridium plugs are def worth it imo. have them in my nc 5.0 and changed them with the coil plugs and leads. idle was smoother, slightly better economy and the engine was more responsive.
coil will do it's job til it dies. it's usually other components in the ignition system that fail, ie rottor button, rottor cap, leads, plugs etc.

Most 10mm leads run the same size conductor, just with extra insulation. IMO just look at some 9mm like eagle, they are decent quality and fairly cheap. NGK iridiums are decent plugs and do give a bit better spark but their main advantage is they last 5x longer than standards which offsets their cost. You want a heat range of 5 (in NGK) or 5.5 (I think the iridiums are 5.5).

Being an EB it probably has quite a few kms on it, for best results you should really replace most of the ignition system including coil (probably around 200,000kms), TFI (100-200,000kms), distributor cap and rotor (100,000kms) but most people don't bother, especially because most of these parts are expensive

EDXR8 is right, go with 9mm leads, i've heard from a few people that most tens are just extra insulation.

I recently did my leads, plugs, cap, button and coil and noticed a big difference. BTW my coil was half gone.......

I got some 9 mm leads done by queensland ingnition leads, champion gold plugs and a msd blaster coil. They are working really well. I also changed over to thermo fans and shoved a k&n panel filter at the same time so i've noticed some performance gains.

I would use manacore leads as there are one of the best around and have low impediance. Also you will only need 8mm leads as they are equivelent to 9 or 10mm leads of other makes.