How can I estimate TL431 output voltage swings based on datasheet alone?
So, the worst-case step response, assuming the thing doesn't ring like a bell, can be had from the impedance of the common-base stage and the high-frequency impedance of the TL431 through it.

\$\frac{26\mathrm{mV}}{10\mathrm{mA}} = 2.6\Omega\$.  By figure 12 of the data sheet the TL431 has a high-frequency output impedance of 12\$\Omega\$ or so.  Assuming a \$\beta\$ of 60, the impedance at the base of your transistor should be 2.8\$\Omega\$.  A step change of 10mA should result in a step change of 28mV, which is a hair less than 1% of your 3.3V.

You might want to replace the TL431 with a fixed voltage on the base of your transistor and see what the step response is -- if it's too great, then the regulator has nothing to do with your troubles.

You might also want to determine if the actual current steps you'll be seeing are faster than the 1\$\mu\$s or so implied by the TL431's frequency response (unless your transistor is REALLY slow).

I think that if this were me, and I needed this to work with super sharp-edged current pulses, I'd have some capacitance on the output of the thing, with the circuit properly compensated so that it didn't oscillate.