Airbrushing Tamiya without primer

I would agree that it may not be necessary to prime small parts that will not be masked or require much handling. 

3000 might be a bit finer than necessary and almost polishes the surface.  Not sure you'd gain much.  Thinning with lacquer thinner helps a bit vs X-20.  Still, it doesn't really stick that well.  I've done some tape tests and it still pulls up much easier than any primer (except Vallejo).

If you mix Tamiya with Lacquer thinner you will get more bite for sure, and always remove some tackiness from your tape, I usually pull it on and off a smooth metal surface like my base fridge.Many builds don't call for a hard masked edge,so that may not even come into play, it certainly handles okay under normal conditions. 

Never a primer, never a problem.

I often use Tamiya + Mr Color Leveling thinners AS a primer....

Also use Vallejo Polyurethane Primer AS an OD topcoat

In my own scratch tests every paint I compared did better with primer, even M&M enamel. But that's not to say some weren't acceptable without primer. Some were outright terrible but others managed. One paint that does nearly equally well with or without primer on styrene is decanted and airbrushed Rustoleum 2x enamel. 2x apparently has some degree of some sort of primer in its formula. Another that does well is MCW lacquer.

So you seem to want to do this, try it is best I can say. Wouldn't be my choice but sometimes you gotta do what you gotta do.

Found out just the other day that MRP lacquers bond extremely well, even without primer.  I have no intention of ever purposely not using primer though, due to scratch and Dymo tape tests I have done, along with just being able to check my seam-filling and sanding work before putting paint down.  It really helps me to see the areas that still need a little bit of attention.

I find I prime less and less and am very happy to spray Tamiya thinned with their lacquer thinner.  Unless the model has a lot of P.E. or metal parts.  If mostly plastic then I "prime" with Tamiya.

Cheers