I rarely play as a GK and dont have a high ath build so I cant really tell how easy keeping can get after a lot of practice but from what I see a well placed throw from minimum distance is almost impossible to catch as there simply is no viable build that gives you enough acceleration/speed to get into catch range quickly enough, also concidering human reaction time. I expect the average percentage of catched shots to be quite low, although I have seen good keepers like shaman but then again seen many bad shooters/throwers.
Almost more problematic though I think is how easy you can push forward. It needs just 3 fast passes to cover the whole bloodbowl field, the first one being uninteruptable as it comes from the keeper. It just takes seconds to counterscore as TDMers randoming along the map, eventually winning a duel are quite likely to be available to pass to. This could be countered of course with teamwork and a well thought player placement but this is, as we all know, not likely to happen in rageball.
I think - 30 % ballspeed would make the gamemode more balanced, as a more limited field of action would require more contribution from other teammates who are actually interested in playing the gamemode in order to achieve victory.