The fact that the groups form faster also speeds up the wait times for dps as well; they just don't wind up with instant queues.
I havn't really got the time or inclination to prove it but intuitively it would seem to me that if you have a situation where a there is a shortage of one role then increasing the player pool would both increase the size of the queue and the rate that group's are formed by the same amount, meaning the average wait times wouldnt change. Also intuitively but not worked out I'd expect increased rate of groups forming would lower the variance in wait times which can only be a good thing
That would only apply when groups are ready to go as soon as the indemand class turns up, in situation's where you don't have that critical mass you would expect the averages to be higher for all roles as when there is no actual queue you could be waiting for anyone
I'm not against x-server lfg but some of the critisism's of it are justified, if the required critical mass cant be reached on a single server then its right to implement a x-server solution, I'd quiet like to see one that prioritises same server matches before looking out to other servers to fill the empty roles