Ok, looks like I spoke too soon. The engine's acting very strangely since the plug change. Sometimes it feels as powerful as it should, sometimes it feels sluggish at 5,000 revs, and sometimes it's fine up until about 6-7,000 revs, but it's just totally inconsistent, seems to be worse once I've switched to the fuel reserve, and is still making a constant rattling sound once I go over 4,000 revs (though it was already doing that before the plug change). Definitely doesn't sound healthy.

flat spot in the rev range that tuning should get rid of.

Have you done an oil change recently? If not then its got to be worth doing one and changing the filter. Also, check that the plugs are all done up tight and that the leads are connected properly.

Also check that the connectors going into the back of the coils are clean and tight, and that the plug caps are clean and a tight fitting on the top of the plugs, mine had corroded so bad they were loose on the plugs giving an on/off sort of feeling on bumpy roads. I fixed it by putting second hand inners in the plug caps a mate give me, New ones would be better though. But it worked.