Thought i'd chuck in my two pennyworth ,when i rebuilt my engine I went down the SRM route simply because a) at that time money wasn't really problem ( doesn't a few years make a difference !!!) b) having been " out " of the bike scene and BSA's for over 25 yrs i didn't think that there would be many local engineering shops around with the interest in working on old brit iron ( how wrong was that but at the time I'd not looked around very much ) .
Having said that 9 yrs ago SRM'S prices were lower AND all the work done was absolutely spot on! I put all the bits together and they were always onhand to give advice and everything fitted well; 9 yrs on and a fair number of miles on the clock it's still in excellent condition .
All my other A'S have been standard and I've only suffered 1 failure in 5 bikes ,that was my first plunger and I'd flogged the living daylights out of her has only a 16yr could and when she through a rod I was going flatout down a hill just outside oxford heading again for london ,if i'd had a little sense I'm sure that that bike would still be running now .
So there you are, basically you pays your money and you makes your choice but either way keep the oil clean ,whatever you use, and only use a company that someone on this forum recommends and you'll be all right, bets of luck BobH.