Piston prices

but tricky to give a good answer!

Some of Andover's pistons are of German origin, so possibly more expensive wholesale than Taiwanese Hepolite.

What is the problem Chris? Piston broke?
Sorry, I couldn't restrain myself.. 

Just read that Seager engineering ( vg engine reconditioners) use Andover pistons, which have Gandini piston rings that are some of the best: oil control rings particularly  well designed. 

..might be a factor. There have been previous threads on that topic which may prove helpful. 
Even been mentioned that a noticeable change might necessitate rebalancing the crankshaft to compensate. 

The ‘news hepolites’ made in Taiwan don't have the right rings for a cast iron cylinder, nor the right clips.
RGM selling the right rings and clips, added to the price of news hepolites it is as expensive as JP pistons (made in Australia) that it also sells.
The OMEGAS are mainly used for racing with a set of modifications on the engine: camshaft, valves etc...

The current Omegas are designed to have a higher CR, 10.4:1 When I had them made it made sense to have the valve pocket widened to suit large valves and on the 850 version the valve pocket machined as deep as the valve pocket on the 750. The top diameter of the 750 +0.060''piston is also reduced slightly to allow for gasket crush when the bore is taken out to +0.060'' as the gaskets are designed for the original max of +0.040'' This eliminates some work when fitting a very high lift cam.
The pistons are forged, and a new forging tool was designed as none existed, until these versions the previous pistons were cast according to Omega. The standard GPM piston which is a casting is heavier than the forged Omega equivalent. Some may ask why not a slipper or semi-slipper? these options were looked at, and when a company that made pistons for a top end European motorcycle manufacturer says otherwise I took their advice. 
We have them fitted to MK3 here and it is not noticeably any harder to start compared to a bike with standard 850 pistons. 
Be very careful when changing a circlip to a different style, the groove and the pin are made to suit different circlip styles. As for piston rings, I once bought a set of Hastings rings, and if anyone wants a set of 750 +0.020'' rings they are welcome, I will certainly not be fitting them though I'll have to try and find them in the garage.  

A question on your 850 Omegas and that 10.4:1 compression ratio if I may Ashley:
is that with a standard deck height barrel, standard, i.e. not skimmed and with squish intact, RH4 cylinder head, and a 0.8mm Victor Reinz gasket?