I think some widely used scripts could indeed be developed into internal CHDK features. But then again, that takes away the user's power to quickly fiddle with the script code (not everyone wants to compile their own builds I think

) for their specific needs... a script that's hard-coded in C won't probably be available as an uBasic or Lua script with the same functionality.
As for speed, I think Lua mostly solves that by not wasting 10 ms for every variable substitution, rem line etc. For example my MDFB script wouldn't get one bit faster to react if it was entirely coded in C (or Lua for that matter), because mx3's md_detect_motion is C code and does the critical part in detecting and (in fast react mode) pressing the shutter. Stuff that happen after shooting, before MD is rearmed would be done quicker in Lua or C, but that's not a problem except in my newest devel version which has Av/Tv/Ev adjustment for a570is (and is not quickly portable for others thank you for asking, it uses about 10 more propcases, button presses (gasp... if anyone knows how to work SSAPI::EnterToCompensationEVF and SSAPI::ExitFromCompensationEVF found in a570is 1.00e firmware strings, please tell me...) and the uBasic modemap which we don't have, at least for models that have negative modemap values we don't).
What I'd really like to see would be an improvement (for both uBasic and Lua) in the script menu, i.e. ways for the script to define it's script menu appearance to a larger extent. Like
- submenus (I use intendation, but that's no good really)
- settings profiles (for this we already have a patch in the juciphox build I think?)
- enumerated+named and range limited input values like choosing feature YES/NO instead of 1/0 and a comment saying what 1 and 0 mean and having to code input sanitation to the script; this we have in several CHDK menus but a script interface to it is missing; could be something like
@param a Funky sizzle
@default a 1
@enum a 0 "OFF" 1 "ON" -1 "You choose" 2 "Not today".
- parameter help pages, maybe like
@help a "Funky sizzle adds the crackle & pop to your holiday photos." - a customized script load menu for 10 of my most used scripts (and their previously used parameters or settings profiles would of course be remembered); I'm embarrassed to reveal that I've never tried the custom user menu, maybe it would be good for this?