I'm genuinely surprised no one has figured this out yet.
I'm dumb, but I'll leave this here for anyone interested.
Go into an invite only session and take your Voltic to the "busy" LS Customs (the one near the center of the city).
Make sure your spawn setting is set to last location in the options menu.
Park your Voltic in one of the yellow lined boxes by the LS Customs garage door.
Now get out and steal a car that can be outfitted with turbo (which is most cars thankfully)
Drive that car into the LS Customs, and go to the turbo menu. DO NOT buy the turbo, but simply put your cursor over it so that it is highlighted.
Now pause the game and go to the Store. Wait until it is fully loaded and then sign out of PSN/XBL and then sign right back in.
After you exit the XMB/dashboard, you will be kicked back to single player. Once you are able to, go back into online in another invite only session.
If done right, you will spawn right outside the LS customs in or near your Voltic. Just drive right in, and the very first menu you will see is one allowing you to purchase turbo. It costs 50000. I have no idea if it actually makes a difference in the Voltic's already rapid acceleration. It feels like it does, however that could just be placebo for me, so feel free to put it to the test if you can.