Hi Voe.
You are certainly right about Fa 77 ltd autofocusing rather slow in street shooting. Often missing the shot by the time lens locks into a focus.
Da 70mm f2.4 is certainly a lot faster if it comes to autofocus speed. You do not get much difference in image quality from this lens comparing to 77 ltd
One thing to note is the issue of using it in harsh lighting where the bokeh does not do as well as fa 77 ltd.
Da 70 ltd also does not perform as well with trees in the background bokeh - rather chalky and distracting much worse than Fa 77 ltd.
Joel has Da 70 ltd and if you are heading out some time, you should ask him for the lens to try out
p.s. Some street photographers I know use Fa 31 ltd for street shooting and does it well. Sometimes it is about how brave and relaxing you are with street shooting rather than autofocus