No problem mate, you did me a favour by making me inspect it much more closely than I would've normally (never checked the plug recess before...in 7 years!)
So, back from a quick 40 mile blast. Report below;
Tickover - definitely improved, with no surging at all.
Low speed/filtering - still naff. Perhaps a marginal improvement but nothing significant enough to recommend the vac port mod
Mid range/acceleration - seems stronger with more torque, although there is definitely a surge/flat spot around the 4.5k rpm area. Probably the plugs and coils account for most of the improvement (if this isn't just me tricking myself).
Top end - great, back to where it was years ago. I'm convinced I had a tiny vac leak from the Scottoiler line that only kicked in under high pressure. Thumbs up for vac port mod
Deceleration/backfire - still there but less pronounced.
Overall I would say the vac port mod is worth doing if you're already taking the air box out for other maintenance but the difference is hardly noticeable. I reckon most of the improvement is down to decent stick coils (mine must've been failing for a while) and perhaps new plugs (although the old ones weren't that bad).
Who knows how long this pair of coils will last, but I will no longer be using my pressure washer at all on the bike.
The coils are installed with plenty of high-temp silicon grease around the seals so will only be exposed to water from wet motorway runs. Time will tell.
Thanks everyone for all the advice, much appreciated...and my numpty status is now confirmed.