Ok so the "the best exhaust in treatland for max performance" is a really hard description to fill. really, you will see power gains with most Peugeot expansion chamber exhausts. that said, cost, ease of installation, and your goals become determining factors.
Right now I'm running the simonini circuit pipe on my pug setup and I love it. this one has a pretty wide hard hitting powerband but it pretty much caps off the revs at 8,000. Allot of folks say it is more prone to breakage but I haven't experienced any of that yet myself so I have yet to see. bottom line, it bolts up nice with no modification and the pedals clear. good pipe at a good price. https://www.treatland.tv/peugeot-moped-simonini-circuit-pipe-p/peugeot-simonini-circuit-clear.htm
If ya want cheaper, then you could get this: https://www.treatland.tv/peugeot-mini-budget-circuit-exhaust-pipe-SILVER-p/peugeot-mini-budget-circuit.htm
I know this pipe works ok with stock but not sure how it would be with a 70 kit...
If ya got a bigger budget, the doppler exhaust or mvt is a tried and true high revy, high power pipe that would work very well with your setup. the doppler will likely require the removal or modification of your pedals in order to fit and is not as budget friendly as the others but it is certainly an awesome huge pipe with a huge personality. https://www.treatland.tv/peugeot-103-moped-doppler-performance-pipe-p/peugeot-doppler-pipe-103-tsm.htm
of course I have to mention the ninja g3 because everyone says its amazing. its very difficult to find in my experience though so unless ya know someone...
Dos cycles also has a lot of interesting peugeot pipes that ya might want to look at as well. https://www.doscycles.com/search?type=product&q=peugeot+exhaust
At any rate, that is just some thoughts to help get ya going. My advice would be to just look around find a pipe you like the looks and performance of and roll the dice, buy it and play around with it. Also I don't have lots of experience with hundreds of mopeds. so what I say isn't gospel... If Andy Scouten, Graham Motzing, Rebel Moby or any of those guys get on this thread, they are the ones to listen to. They have some great knowledge and will give ya good advice.