"I heard from Mike Beez " lol
you absolutely need to upgrade your pipe. I've personally been a big fan of the Alsil Quich pipe for peugeot SPX. Theyre cheap and rev high, and fit really easily on the sachs. you need to derestrict the header and baffle on the pipe as-received but that's a simple task. you may need to slightly file the clamp mount and drill holes in the bottoms of the fins on the just to attach a pair of spring mounts, and then hose clamp the back to the usual exhaust mount.
The gianelli sachs pipes blow goatnuts and fall off and are pretty worthless anyway
You can remove the baffle on the stock D pipes and do fairly well as far as just keeping it derestricted as you've noted, but it's loud. you also get no powerband, but at least it's not holding you back.
You can weld or clamp on a different expansion chamber to your existing stock header.
There's a review of the pipes on 505/1a performance in the wiki
Also, with the flat port bolt-on exhaust option on the kits, you can also bolt on a number of alternate pipes including the hurricane blaster for garellis. this is my favorite pipe ever made for kitted bikes and puts out ridiculous power, though they break pretty easily.