I've heard of many sorts of media being used in a reactor, foam, bio balls, ceramic noodles, etc. but basically the denser the media employed the more it'll slow down the flow down just the same as it does inside the canister of your filter.
I personally do not use media at all in the reactor chamber and as a result the loss in flow is negligable although undoubtedly there must be some. I do get a few tiny bubbles being spat into my tank through the spraybar now and again though but it doesn't bother me at all. On the contrary I quite like the effect and its really easy to tell if there is to much co2 being injected as the frequency of the tiny bubbles being spat in through the spraybar increases giving an audible warning, I also quite like a few tiny bubbles bouncing around the tank as it aids in understanding the flow dynamics within it.
I did try my reactor with bio balls and they do help disolve the co2 more efficiently but the balls tended to make a bit of a rattling noise as the top ones in the chamber were bounced around in the flow. Tiny co2 bubbles did lodge/cling onto the balls which obviously results in the bubbles being in contact with the water for longer so aiding the co2 to disolve. The larger bubbles also break up when the flow pushes them against the bio balls.
Writing this a solution has occurred to me that maybe if you were to combine somthing to stop the balls moving around above them in the chamber .i.e. foam or some heavier media then the balls could be used without the annoying rattling noise. Thats somthing I may try out in the future, come to think of it I think I've seen this arrangement somewhere before.
I can't imagine how noodles would do the job any better and I've never tried them so can't comment from experience but I would like to understand the logic behind there use.
Regards, Chris.