Increasing queue depth increases latency although it might avoid drops. I.e. if your D1 class application has slow response to lost packets, and you see queue drops, increasing queue depth may help. If your D1 class application has slow response due to queuing delay, increasing queue depth may not help at all.
You could also try increasing bandwidth allocation to your D1 class applications (to dequeue such traffic over other traffic). However, if that traffic actually needs more than about 2/3s of your link capacity, you might not have enough bandwidth to meet that application's service needs.
(NB: for maximum TCP performance, interfaces need the queue depth to support about half the BDP. 64 packets is often a default queue depth useful for "typical" WAN of T1/E1 bandwidths.)
On some/many platforms, software queuing only "engages" when the interface FIFO queue overflows. Sometimes you also need to decrease the interface tx-ring-limit to get the bandwidth guarantees you've defined.
