The reason to have default minimum 5us bulk delay is because the digital circuit of bulk delay introduce some fixed delay per path and per MIMO radio link. To align them together introduce about 4us delay for all.
There is a way to have smaller bulk delay for the channel you are interested without enabling bulk delay. Assuming you would like to add 1.67 us bulk delay for all the radiolinks of the channel, you can apply additional 1.67 us on top of path delay for every single active path of the channel. Please seen attached graph as example.
Example shown in attached diagram is UMTS-PA3 channel model. By adding 1.67us on all four paths I achieve equivalent 1.67us bulk delay. Hope this helps.