I have no experience of using protech shocks but having had a look at them I personally wouldn't buy what is claimed to be a brand new, custom made shock for the same price you have had to pay to get your existing damper reconditioned. You've already experienced the results of trying to sort this issue on the cheap.
At 115KG you are asking a lot of the suspension and outside optimum design spec for the OE suspension.
Have a look at brooks suspension. Marc is very clued up and offers sound advice but you need to know what you want and are trying to achieve.
Springs are relatively simple, dampers less so and can use different methods to control the oil flow hence differing levels of adjustment, complexity and cost, Bladder vs emulsion etc.
Having run Ohlins and Wilbers in the past I am now using Nitron as supplied by brooks and I have been very happy with the both the service and the performance of the shocks for the price and think they represent good value.
Nitron are a UK company so in theory the support should be at least as good as the likes of Ohlins etc but I have yet to have to deal with a problem or have a shock serviced.
While I would agree that the rear shock has the greatest impact on handling, if replacing the rear with an aftermarket custom tuned shock then I would also change the front to match as the original front will be an unknown and at odds with the set up of the rear. Having run a GS with a stock front and custom rear I deffinitely did notice the improvement in replacing the front but then I am relatively light, ride mainly solo and ride'progressively' at times as opposed to just bimbling along two up.
Ultimately it's your money and your bike, it's up to you to decide which route to go down.