I installed a set on a gently-used bike last year. I think it has about 500 miles on it now. Be sure to use the clearance of 0.0010" to 0.0012" wit those pistons, as they do not have the micro-groove skirts like some 836 kits.
If nothing else is done but boring and installing these type of 836 pistons, the typical HP increase is about 8. If you follow all the steps in my book, like valve work, port work, quench band, spark curve change, and final carb tuning with stock airbox (NOT pods), you can get 12 HP from the installation. If you also add a mild cam like the K0 cam, this can add 2 HP more to that figure. If you go much further than that, like a Webcam 41 or something, you might wish to consider stronger rods as you nudge toward 80 HP. MRieck can install larger intake valves for you, too, which can push it to a 90 HP and require that you think about something other than a stock spoked rear wheel.