RO, If you are trying to get fast / linear throttle response changing to a digital servo will not do much.
Try learning to tune the engine not just set the needles. Alot of people believe that if they set the low speed
needle to a good idle and the high speed needle to slightly rich that the engine is properly set. That may be true
for trainers and sport flying but as you get more experienced and want a linear feel ther is more to tuning.

First you need to setup the throttle servo to use maximum servo travel from idle to full throttle. This is
done by adjusting the linkages and setting the EPA. ( I use 0 epa at idle and 110 at full throttle / Neg for kill)

Next start by setting the needles to where the engine runs good (basic full throttle and idle pinch test). now
fine tune the adjustments by running the engine through the entire range. You will most likely find the engine
does not respond well at idle to 1/4 throttle and again at 3/4- full throttle. you may need to lean or richen the low
speed needle to get a smooth crisp low end transition and the same with high speed to achieve a good top end
transition. these adjustments will be very small. redo the pinch test and keep adjusting until you are happy.

Midrange is another story as this is a mixture of high and low speed needles, combined with backpressure
and exhaust design. OS engines usually have a good solid midrange. However some of the less expensive
engines have a less desireable midrange. the only way I know to get a vey good midrange is to put a pipe
on the engine and tune it properly. however I have been told by many competition pilots and have some experience
with PERRY carbs and they are easy to adjust and dramatically increase the performance and ease of tuning
for some of the less expensive engines,