I use one of the Alturn USA servo's in my Centro and have never had a problem with it and its quite quick. It's nearly £40 so its a similar price to the Savox one u mentioned. Here's a link to it
http://www.modelsport.co.uk/alturn-u...roducts/359790.
I've also got the other high speed one they do which is only £18, I'll have it with me next time if u want to try it out. This is the one I mean
http://www.modelsport.co.uk/alturn-u...roducts/359785
Hope this helps Mark