Our region had (still has) a 06 Rotax 125 that we used last year as a recruiting tool. Interestingly enough, not a single recruit drove the TAG became a regular, but we grew from 7 regulars to 13-14 season subscriptions this year, and we expect somewhere around 20 per event once the season starts, so it might had some effect.
You will lose a lot of time at the start in a Rotax. You simply can't hammer the throttle and expect it to take off and subsequently you will lose about 1-2 seconds at the launch. Once it's on the way, you would actually pick up time if you are not familiar with shifting. Like Tom said you can actually concentrate more on driving instead of driving plus shifting, which could make your time faster.
Once you are used to shifting, then there is no comparison. A shifter will be faster than a TAG in autocross. We have a very good driver locally who ran the TAG all year last year, and he would consistently beat me in just about every type of car or kart, but I am generally 0.5 - 1 second faster than him in my shifter vs his TAG. In our T&T I have ran the shifter and TAG back to back and the shifter will always be faster.
Depending on the setup, a lot of TAG do not run rear bar or front bar in sprint races, which is somewhat of a challenge to drive when the tires are cold in autox conditions. You can easily setup the kart similar to the shifters, make everything stiff, and get some sticky tires. And you should be ready to go.
I can share with you some of my setup information. Please keep in mind I am a mediocre driver and my setup is probably not worth much, but it will get you started.
Vega yellow when it's cool, MG green when it's hot.
Start with 47" front width and 54" rear width. Widen the front to get more front grip, shorten the rear if you have exit understeer.
Run front and rear torsion bar, put the 4th rail on if you have one.
Most folks set the ride height on the high side. My ride height is in the middle because I am taller.
Front and rear weight bias should be somewhere around 43/57 to 40/60 range.
Toe out in the front.
Run at least two seat strut and add more if you like.
I don't run any camber, but I do run with more caster.
Setup your seat with the bottom of the seat the same height as the bottom of the chassis tube. Some folks run a little lower, but I don't think it's worth it to scrap the bottom.
You don't have front brakes, but I think this year you can add them without weight penalty. So definitely add front brakes if you can.
If I remember correctly, we ran 86 tooth rear sprocket for autox for a top speed of 72.
I am sure the manual can tell you better, but I think we ran 24:1 redline synthetic mix with 93 octane unleaded.
I think this will at least get you started without spinning everywhere. If you look for previous posts from Alan and Brian both here and EKN, you will get much better setup information than what I have. I learned just about everything from Brian and stole ideas from Alan. But chassis setup between a shifter and TAG should be about the same.
P.S. Make sure you charge the battery fully before every event. It's not fun when you can't start the kart because the battery is not fully charged. And keep the chain well lubed. The sprocket and chain don't do well without a lot of lubrication. Follow the Rotax manual for jetting and gas mixture and you would be just fine. Have fun!
DK