Brought Dart to shop to fix idling issue and car runs like s**t!

Cuz it's a racecar,don't you know,lol.
That said;
There might be a legitimate reason, IDK, to defeat the V-can, but, in a streeter I haven't seen it yet. I suppose that if your engine couldn't produce enough vacuum in the intake, to operate it, in the rpm it needs to work in with your gears; that could be considered a legitimate reason. But IMO, that would not be a very streetable engine. Cuz, even the 292/508 cam in my 367combo, pulled enough vacuum by the time the throttle was opened far enough to activate the port, to pull in all 22 degrees.
OR
it might just be that the builder has not yet had time to tune the Vcan.

But, it could also be; like the tuner kids who put KnN Cone-filters on their fart-canned Cavaliers, but then it doesn't fit in the stock air box, so they just let it suck hot underhood air..... totally defeating the purpose. And nearly everyone of them that I have talked to, swears how much faster their car is now.

But hey, it could be even worse;
some of those tuner kids have told me that the filter is worth "x"hp, the fart can is "y"hp, the resistor they Tee'd into the IAT is worth "z"hp, and so on; so in the end they, adding it all together; x+y+z= they doubled their horsepower;
don't you know.....lol