Commando 850 MKIII - vibes only below 2500 rpm, is this typical?

While the Isolastics are good, they can't mask a rough engine. Lumpy low-speed running is most likely carburettor balance, wear and adjustment. And even though you've said it's perfect it's worth checking again.

Then play with the tension in the head-steady spring. The manual tells you roughly where it set it but a bit either side often improves matters. And set the Isolastic clearance to the specified 10 thou.

Isolastic rubbers do deteriorate over time, which means the engine isn't suspended as nicely as it should be, and some of the replacement rubber bushes are less supple than they should be.
See https://www.nortonownersclub.org/forum/isolastics-answer-low-rev-vibes for some suggestions.
Plus a lot can happen to a bike over 50 years.

Having ridden a brand-new Mk3, straight after its 500 mile service in 1977, I can testify that they can be made to deliver an impressively smooth ride. But there's always a bit of vibration down low, some of which can be identified as power pulses from the drive train.

Even without vibration, you should avoid lugging it along under 2000 rpm in any case - it's not good for it.

The vibration you are experiencing is "torque induced" vibration, its source, as Stan says, is from the firing impulse, assuming all is well with the engine.   No amount of rubber mounting will get rid of those and be able to deal with the out of balance mass vibration.
That's my suggestion based on very little experience of the Commando let alone the isolastic system!
I've been lucky enough to having ridden several Commando's, the most impressive was a nearly new 850 in the late 1970's.  They all vibrated  until the Iso got to work, somewhere between 2500-3000 rpm, because of the above.
 

The Mk3 with its bonded rubbers and threaded adjusters does seem to have the smooth patch higher up the rpm range. 

It was also apparently the case that for a considerable period, post-production replacement rubbers were of necessity made of a harder rubber which transmitted more vibration. It's impossible to know what you have fitted. 

As a non Commando owner, I would like to propose a point, I have dealt with quite a few 4 stroke engines that have improved in a lot of ways after rebalancing ie as the engine came from the factory its balance factor was a bit 'casual'. Could a specific Commando engine benefit from a fresh rebalance?

Very good answer-take note all you Commando owners winging about vibes!
But equalising combustion chambers and gas flowing will keep you busy for a week or two......