Revs Racing told me (and I don't have a reason not to trust them) that the OEM ESA shocks cannot be rebuilt.
Perhaps they meant they cannot rebuild them.
Having modded suspension on many bikes I have found most can be rebuilt by someone somewhere, and one company may have found a way to rebuild something another company cannot.
I have used MH Racing, Maxton and MCT (plus others) over the years and where Maxton did not want to touch my ZZR rear shock MCT did a grand job on re-working it.
The terms rebuild / refurbish / rework / re-condition / re-valve can also mean many things to many people.
A refurbishment could mean chainging the oil, or maybe also re-gassing, possibly include replacing seals. If someone changed my engine oil I would not consider that I have had my engine "re-furbished" or "re-conditioned".
None of these terms have any real meaning, but I would say the majority of small businesses in this market are pretty honest.
MCT are my preferred supplier as I have used them a fair bit and always had honest advice, fair prices and excellent results, you can also sit and watch them work.
If going for any sort of re-work find out what you are getting, in my experience just re-gassing and re-oiling an OEM shock can do wonders for a relatively low cost, and is often a good option, but if seals are unavailable and the shock is older I would be more tempted to put the cash towards another unit as opposed to risk having the work negated by a blown seal a few months later.
Quality aftermarket items (Ohlins / Wilburs) are always going to be better than OE units, even re-worked OE units, but obviously at a premium.
I have often needed a wider range of adjustment, or wanted something the OE unit does not have (such as a remote pre-load adjuster) and if you need a different spring as well as a service of the OE unit the cost will be creeping much closer to that of a new shock.
Also consider the re-sale value of an Ohlins / Wilburs which will be 50%-75% of new cost - you will not get any of the money spent improving the OE shock back when you sell the bike.
I consider my machine a mobile savings account

and I my reckon pension fund depreciated more than my bike last year
