Your bike would have left the factory with a 67-334 cam which would suit its original compression ratio and broad torque delivery. The 67-356 is a later step up in power but keeps the low end torque. I have a 356 in my '59RR. The 357 is the last cam with the most power, there is some debate (on-going?) about whether this is "peaky", opinions vary. My take would be that in most engines when the power is upped by a cam it ups the power in the higher rev range by helping the engine breath better at high revs, at mid range and low range there's not much difference.
So if you intend to keep the bike in the upper rev ranges then you will feel the improvement from the 357, but if not then its not a benefit. A10s are not a high revving bike and certainly with a Golden Flash the great pleasure is opening the throttle at 30 mph in top and pulling up to 70 without a gear change. If you look at period road tests they are not much bothered by 1/4 mile or 0-60 but they talk a lot about 30-50 and acceleration in top gear from low speed. Its a different riding style which about getting into top and staying there. If you are riding with a manual A/R this makes sense.
I did a lot of miles on a tuned (10.5:1) A65 and it was very revvy and peaky. I liked it but I liked the gentle power of a softly tuned A10 too, it was more relaxing and comfortable. If you want power and revs in a BSA just get a twin carb A65, they are fast, cheaper than A10's and there's plenty about. Don't spoil an A10 trying to make it into the A65 you should have bought.
