Consider a SOA design implemented in .NET running on the same machine, where dynamically switching off, unloading, loading and restarting components is obligatory. (The efficiency of this procedure is not part of the question).
Of two options : Multiple Windows Services or Multiple AppDomains "in a" default AppDomain, taking into account the fastest possible communication channels in both scenario´s, when it comes to communication speed: Is there any difference?
To the best of my knowledge the difference in speed between the two methods will be minor. The only way to tell which will out perform the other is do a real world test on a machine configured like your production environment and compare them.
Most likely you will find the RPC method you use will not be the bottleneck but something else will be instead.