I guess I will be the stick in the mud and ask, because this is a beginners forum.
What airplane are you flying?
What is your experience level?
The reason I ask, again this is a beginners forum, most beginner and second planes would have plenty of authority with the servos you have. In fact at 4.8v you probably haven't flown the airplane to its max potential. Going to 6v shouldn't make any difference at all UNLESS you are flying something very aerobatic and with enough skill to be able to tell the difference in any servo torque/speed.
The answer to your question is yes it will improve performance of the servos but it will also reduce flight time on the battery pack.
PS: another thought. Those servos are 69oz at 4.8v more than enough for anything being flown by the majority of the people on this forum. More than I have in my 1/4 scale RV-4.
Are you flying Giant scale? I do have high torque servos in my Showtime 90.
.18 to.22 on the speed. are you going to be able to tell a 4/100th sec. difference?
All that said I am not against using a 6v pack. I just don't want you to get the wrong impression on what it will do for you.