A 2028 is more than enough filtration and I would think current for a 28 gal tank. It holds more media to sustain bacteria than you will ever be able to utilize in a tank that small. Remember, bacteria size is proportionate to the tank size and bio-load that it will produce. Once a colony has been established in the 2028 no more will produced.
Adding more filtration in the way of a sump wont do you anything. But add current, you could do that with a powerhead. Personally, I dont think you will need more current, the 2028 will probably have to be restricted as it is.
Ehfipimp #273 ( Eheim Classic: 2260, 2217, Eheim Ecco: 2236, 2232 ) Fluval FX5