Well, no doubt the 909X would benefit from a longer antenna, but I've put the reel antenna aside, because it brings in more RFI than the whip does. The main difference I experience without the external antenna is on battery power compared with AC operation. There's a big drop when I disconnect the adapter, not only in signal strength but in volume, as well. This is most noticeable if I listen out on the balcony, because street noise from cars going by makes it hard to hear the radio. This affects HF more than the other bands, and I've seen the battery indicator drop a little when I switch to SSB.
My guess is that the adapter delivers more power than the batteries, even when they're new. Maybe this is the source of all those complaints about poor sensitivity with the whip. I mainly run the radio on AC, but for portable use I favor alkalines, as they supply 1.5 volts, whereas the average rechargeable penlight battery puts out only 1.2v.
As for the DSP, the 909X circuitry probably differs somewhat from that of the 909, but I'm no expert, either.