Hi,
Overall with the capacitor mods, there was a few percent improvement in rise-time, especially with one or more capacitor(s) on top. The additional capacitors next to the chip, i.e. soldered on the PCB, improved rise time by a few picoseconds.
Scouring the web, it seems most of the 74AC based projects are reported to be in the 500-700ps ballpark.
There's lots of interesting things to try (for instance different packages, separating out the logic into single-gates devices, adjusting delays to smooth out the current demand, and so on), but it's diminishing returns if I stick with the current circuit and PCB and trying to make tweaks to it, since the circuit is already well within that ballpark.
It depends what performance you want. As is, it's already about tenfold faster switching compared to old boatanchor HP pulse generators (of which I own one, but it covers a large portion of desk depth whenever I want to use it, and it's noisy). Another interesting thing would be to attach your circuit to (say) Pi Pico (or whatever you're comfortable with) and then you've got the ability to program pulse trains (e.g. double pulse using the Pico PIO to make it very deterministic, or specific pulse widths and so on, depending on requirements).