pbc0, I appreciate you've developed a cable operated system for the RC51 from parts off a Honda Varadero and feel that for you it's the best solution, but l think there's no need to go into how a hydraulic clutch system works compared to a cable operated one, unless we really want to compare the pros and cons of them...(I guess it might be interesting to, though - so I'll discuss below)?
What I'm saying is that there's nothing really so bad about the standard SP2 hydraulic clutch if it's in good working order. At least I never felt anything wrong with mine over a period of 7 years. It didn't grab too early, nor late or feel as if it needed improvement. Perhaps SP1's are different? - I don't know as I've never ridden one. Perhaps someone else can chime in and let us know their thoughts on the SP1 system.
TBH I changed my clutch master only because a good used billet master came up at a good price the same time I bought my forks and it's looks would match my Brembo m/c better so I went for it. It's slightly larger bore and yes it requires slightly more effort to release the clutch as a result & yes the effect is reduced "feel" but even in traffic where I need to slip the clutch accurately I find it doesn't really bother me. I wouldn't have minded at all keeping the standard bore size either.
Personally I prefer a hydraulic clutch release system since it's less troublesome & requires less maintenance over the long term. I've had plenty of bikes over the years with cable operated clutches. Cables stretch over time, need lubrication and if the adjuster at the lever isn't properly set it can cause the cable to chaff and begin to break. If you ever run out of adjustment on a cable system (Because the cable wasn't 100% right to begin with) then it becomes another problem to resolve. Also the OEM lever mechanisms on cable clutch are of typically of lesser quality (They come from cheaper to manufacture bikes after all).
There are pros & cons to both types. Hydraulic system components combined are slightly heavier and may feel a little different at the lever, but generally they're smoother to use since there isn't the friction of a cable inside a sleeve and pistons inside cylinders are effectively always lubricated...
Let's look at the simpler form of brakes on a bike. Would you prefer the precision of a hydraulic system to operate the brakes over a cable operated one?
Me - I'd go for the hydraulic one every time. Because there's usually less "play" and less wasted effort.
For a clutch on every down change I'm pulling in the clutch lever and rev-matching, so why would I want to introduce play & added friction into the system when it would only make the system less efficient & quite probably more trouble prone?
I guess it comes down to preference. I daresay on a race bike where you're shedding the last few grams wherever possible a cable operated system may be preferable (& any decently prepared race bikes will tend to have a quick-shifter & slipper clutch), but on a road bike? With all things considered, I prefer hydraulic.
I think most owners would agree there's really no great need to change the original system as it works fine if in good working order (Makis' experience of changing the slave cylinder seals sounds like good advice). Like anything on a bike, poor maintenance can mean performance can be compromised that's all...it's not necessarily the reason to give up & change a system in it's entirety.
Euro RC51 SP2: HRC WSB Ti Hi-Level, T1 Airbox & Snorkel, PCIII, Dymag CA5 Carbon Wheels, Ti64 Spindles (F, R & Swing Arm), Mori Link, Maxton GP7 Shock & Ohlins 832 Forks, Brembo GP Calipers & SBK narrow track (F), Braketech Disc (R), Brembo billet Underslung x 2 Brembo RCS, Tyga Triple, 7075 Stem, Probolt Titanium all over, Babyface rearsets, MR Complete Carbon Lineup, Custom Undertail & CBR 1000 LED, Watsen LED Indicators, Harris Brace, Giles clip-ons, Corbin seat, etc. 2013 BOTY.