I-140 is not dependent upon PD (date of labor). Having said that, they go by the date on which I-140 is received (RD).Say if you apply for I-140 now and is received by them on Mar27th, then your RD is Mar 27, 2005.
I think, for non-current cases I-140 is faster than that of concurrent cases.
However, for concurrent cases I see backlogs from nov 2003 onwards. this means there are people who have been waiting for more than a year for I-140 to be cleared.
Apart from this, 485 is your application and you have to pay most of the expenses involved and I do not see why they are worried about I-140 being rejected causing problem for 485. Try and talk to them to file 485 together with I-140 as this gives lot of peace, as sometimes unexpected retrogression issues could delay filing 485.