The little I have seen, that is generally the case. DD-WRT, Tomato and Open WRT generally perform at least a little worse in terms of range/speed than the firmware that comes on the routers.
You do get a lot more features and possibly better security (maybe/possibly/ DD-WRT doesn't seem to have a great track record of closing vulns and a lot of distros for older routers haven't seen much in the way of updates in a long time).