So, did Pete give you the dyno printout that shows the air/ fuel mix and which, I would presume, spikes around 3000 revs? Did he say you need to change the jetting, or did he say it's a minimal spike and just ride through it?
In my opinion, I think the person whose said that an x2 isn't compatible with a Lambretta is just confusing the issue for you. I've seen plenty of Lambrettas with x2 needles set up on both AS266 and AV266 atomisers.
Having said that I can't believe your set up is that far wrong, most TS1s on a PHBH 30 will have pilots ranging between 50 and 58.
If you're not going to take it back to PSL, trying a 55 would be an idea. Just don't wang it at wide open throttle and then drop off the revs quickly
. Just go up and down the gears and see whether it responds better. If it runs a bit better you should then try dropping the needle clip down 1 to richen it slightly mid range to compensate for the weaker pilot.