Oil consumption with reasonable cylinder compression in high millage (or Km) can also be the top rings moving up and down in the ring grooves, which tend to pump a small amount of oil up to the combustion chamber with each stroke. If you remove the barrel, squeeze the ring all the way into the groove with your fingers and see if the ring can still move up and down noticeably in the groove. If it does, you need new pistons as the ring has hammered the groove wider. This is why high revving engines have thinner rings to reduce the effect of inertia at the top and bottom of each stroke.
If you remove the head and look at the piston top, if there is a clean area on the thrust and opposite side, with carbon in between, then oil is being passed up into the cylinder by the rings fluttering in the grooves, which removes the carbon.
This cylinder can still give good compression if the rings to bore clearance is still good.
The cylinder that is showing only just over 105 psi is probably leaking past one or more of the valves (seats). Although it is not bad yet, it will only cause a slight power loss at lower RPM only, but if you have the head off, it is worth re cutting the valve seats and valve faces etc. Also check the valve to guide clearance. A certain amount of wiggle is acceptable, but too much will allow oil down the inlet guides under high vacuum, but not so the exhaust. To check this before dismantling, with the engine hot, go down a hill with the throttle closed for some distance. Then blip open up throttle and looking in your mirror, if you see a puff of blue smoke (not black! too much fuel!) then the inlet guides/ valves/ seal are worn. This is best observed by someone following you and filming if possible.
If you replace the pistons and go oversize, depending how big you go, weigh the original piston complete with rings circlips and importantly the piston pin and try to make the new on the same weight as accurately as possible.
This is to prevent upsetting the engine dynamic balance, which can cause problems.
As the new piston is likely to be heavier, equalizing the weight is best done by removing material from the piston pin, as steel is heavier than aluminium. Do this not by boring out the inner bore as this will weaken the pin, but tapering from each inner bore end so as to create a small cone appearance. This as long as it is done in moderation will not affect the pin strength and is often how the manufacturers do it.
Do NOT remove metal from the pin ends as this will allow too much pin end float which will be catastrophic!
Lastly, if fitting new piston/rings, run the engine in for 250/500 miles on mineral oil, NOT synthetic or part sythetic, as the qualities of synthetic oil will prevent the rings from ever sealing with the bore and it will always burn oil!
After running in fully, then use Synthetic oil !
Hope this helps.
PS, If you do change the pistons i would be interested in knowing the standard weight, as i may do a big bore engine
in the future.