Indeed a lot if not the majority of CRT can benefit from a different bias for the spot size (for a better one), but I believe some ones having already good focus, and it can be "too good" especially for normal resolution TV's.
I remeber old_tv-nut mentioning this issue in this thread:
http://videokarma.org/showthread.php?t=276967
About normal TV having incredible focus/spot size for the given size of phosphours, is possible to saturate the center of the ones, if I understand well, generating different color saturation.
I tempted to make this with my Sony F99 chassis monitor (and it for sure will be benefited by), but the G2 in these are monitored and automatically controlled by the µcontroller and a AKB system, amplified by a HV low current transistor. It blanks the screen if one messes with the voltages too much.