First off, There is no way a sidewinder will get outrun by either of those sleds that are beating you even if they are modified and have nitrous on them if they are true trail sleds, so we know there is a problem with the sled. Now on the other hand, if your racing a trail sled against a real "drag sled" then yes, they will murder you even if you are tuned up...
I've had many in here that leak and show huge improvements once sealed up. You can pressure test till your blue in the face and it will not always show the leaks. Typically when pressure testing is done when the machine is cold, and this should actually show the leaks better, but it won't show the leaks that are there once warmed up. Most do not test under high pressure either. You need to block/seal off the exhaust pipe at the turbo and clamp off the valve cover vent and pressure it up to over 30 lbs. to check for leaks properly. I always see the 6 clamps leaking behind the throttle bodies leaking and they usually leak bad. The spacer on those clamps need to be cut in half to ge them to seal up. That rubber gets soft when hot and leaks. It can't hold the pressure with the stock clamps no dealt with.
Try changing plugs too. Ive seen plugs actually go bad but not misfire. There is an issue somewhere that needs addressed.Did the dealer that put that cap on the wastegate valve and not the airbox with the bulletin to vent it? Lots of Dealers have been messing this up and getting it backwards so it won't boost properly or high enough. It's an easy fix for a guy that knows the sled and the way the systems work. Is is slipping the belt in the secondary? If there is not enough side pressure on the secondary they slip and waste away the power. You don't the old V1 Dalton Black Orange or too much helix as it will slip. Use the V2 spring wrapped up around 90 for a sled with tunes. It needs side pressure if you have traction. After much testing I found you need the Heavy Cat spring or lots of wrap in the V2 spring. These sleds make lots of power and if all is not perfect, they can and will waste it away thru leaks and clutching and be inconsistent. I've seen tuned sleds that are slower than stockers instantly because of the leaks and improper clutching.
They are not for everybody, you must have a good tuner on speed dial or have the knowledge on how to make it work or it will not be fast. I always say these sleds are for the experienced tuner. Mine gave me fits more often then not until I got it all ironed out and evolved to the point it is now. There were times I hated it and wanted to run it over with a Dozer! Seriously! It was only engineered for 200 HP and were pushing them well beyond that. The issue for me is all the electronics and hoses and clamps on the machine that leak or could leak. Its insane! Nothing like pushing boost to a simple Apex or 1200 Doo with mechanical boost control. We have electronic control of boost thru the ECU which I don't care for, but it is what it is. Who wants to ride an Apex or 1200 Doo anymore after riding the Winder? LOL!
