[QUOTE
EDIT : my 'distances' are very accurate - i have seen most modern cars that display higher speed than actual speed (and therefore higher mileage) - when my car says it is doing 30/40/50 or whatever mph - GPS agrees with it - it can be perhaps 1mph out...[/QUOTE]
Also run the GPS for a while and check the odo. For example my miata is 5% off on the speedo as most miatas are, but the odo is dead on. There is not always a direct relationship between the odo and the speedo.
I think the improvement of 10% can be blown away with EOC and proper timing. However adjusting timing by turning the distributer without the vacum advance hooked up and then hooking it up will show the best gains.
Also one more thing I thought of when I bought my HIF44 carb they were saying that some of the SUs have a contact needle where with every opening and closing the needle rubs against the jet which will eventually enlarge the jet. I did a quick search and didn't find info about that for the HS4 but found that there are two typs of jets, red and waxstat. Red is simpler and for performace and waxstat is more economical as it is temp sensitive and will run more effeciently on a cold motor. You also said it had a stage 1, which usually has a needle to run rich, at under 10GBP a stock needle will probalby pay for itself pretty quickly. The waxstat jet at 24GBP is probalby only worth it if you know it is worn out (enlarged) or if you have a red one on. Since EOC means you won't always get up to temp quickly you could see it pay for it self soon.
Chart, Carburetor Needle - Classic Mini
This lists different needle and spring specs. The spring as well as the oil you use is important. If the oil is too heavy it will slow the closing of the carb when you take your foot off the gas.
And a nifty comparison in graph form
Minty SU Needle Compare-o-rama I pulled up ADE AAU AAM AAA and AAV and it clearly shows that ADE and AAV are the leanest of that bunch common to HS4s.
Also if you don't have one at about 10-30GBP you can get ram stacks. This will improve the flow of air into your carb which would mean you would have to use less throttle to get the same acceleration, a faster air flow over the needle and better fuel atomization.