I've not done a 'swap' but do have both a 2008 efi (a B5 I think but never too sure on the model bit) which I've had for about nine years and added a 2019 Himmy last month. I've only done 600 miles on the Himmy and bought it for green lanes more than road work but so far I'm pleased with it. About as different to the 500 as a single from Enfield could be, I'll list a few bits I've noticed so far:
Much, much smoother, which is nice but doesn't have the same 'thump'
Modern construction - you can see it's been designed from the ground up rather than bits added, removed, wedged on over time, for example no silly mismatched nuts and bolts, electrics routed better, etc
Not quite the same performance as my slightly breathed on efi (exhaust, K&N, Power Commander, larger front sprocket) but the HImmy feels like it will hold an indicated 70 (if required) forever whereas the efi not so much!
Brakes better, I fitted EBC pads to front from day one, ABS if that's your thing, handy with knobblies on these cold, greasy roads, not being able to lock the rear bit of a pain off road but not a disaster
Nice dials and info without being silly, easy to read dials and enough LCD info to be useful without distracting
Compass is hilarious
I've fitted a YSS rear shock as 6ft+ and 16st in kit and I want it for off road use, I think original and front forks are fine for road use and deffo more comfortable than the efi.
Handling seems neutral and, even on Conti Twinduro tyres and the cold, wet roads has been fine, I'm expecting dry road handling to be easily good enough, maybe even a bit of fun. I've always found the efi ok (Hagon shocks help) but our pock marked roads and short travel suspension make it pretty jarring.
Nice riding position on the Himmy, seat a tad low for me but I've not done any long stints yet, the 2 hrs I have done it was fine so will wait to see if I need to add a bit more height for my knee bend.
On the road it pulls along quite nicely (I've only just finished the running in on mine so it will probably loosen up a bit yet) and will indicate 75 or a bit more if you really must, 55/60 seems comfortable cruising speed which it pulls easily. Would probably take a larger front sprocket if road use only.
Clutch is light and lovely feel for pottering and off road, good standing position off road, it feels like a proper trail bike.
Front mudguard spits a fair bit of gunk off the road even though I've fitted a Hitchcocks extender, I do have 10mm riser on it and Conti knobblies so perhaps standard set up not so bad, anyone else?
Screen is too low for me (6ft tall) and puts wind flow straight onto my face, not great as I wear an open face helmet. I'll probably remove as really not a fan of screens on low powered bikes, I managed plenty of miles on much more powerful 'naked bikes' (what a silly name) without a screen so not really needed IMO, a styling thing
Switch gear is simple and easy to use and it, and all the electrics have already survived a total submerge when I dropped it in a massoooove muddy 'puddle' first day out.
The front racks are handy for helping haul the bike upright and out of puddles, and great for stashing bits on, as is rear rack.
The three tankfuls I've used have given mpg between 55 and 74, variation due to running in throttle use on road, low gear use green laning, I'm guessing 65/70 mpg will be easy on the road, a lot more if you potter. Not as good as the efi which, even two up and giving it a bit of beans, usually gives me 80+
One observation, the Himmy looks like a Landrover, the efi (chrome tank version) looks like a proper old Brit bike and I've already noticed fewer older gents commenting ..
In word, so far I really like the Himmy, it feels different to the efi of course but still has that Enfield dna and feels like it will be the go to bike for back lanes and mucky weather pottering - I currently have a DCT Africa Twin for that which is a great bike, but somehow the Himmy has wormed its way into my soul already, a proper Enfield to this relative Enfield newbie.