There are a couple ways that it seems that you can squeeze out a few more microseconds. I ran some benchmarks for size=1000, and on my 2.4GHz P4, I was able to trim the execution time from ~0.76ms to ~0.70ms.
First, get rid of the coercion dots by converting those two coerced wires to dbl.
Second, I read somewhere (probably one of the Labview App notes, which I can no longer find), that case structures have a fair amount of overhead associated with them, so for simple things like your Matlab switch, it's better to use a Selector node.
When I decided to add the MATLAB switch, I was no longer woried about a few microseconds--users who are that pressed for performance can extract the portion of the diagram they need and remove all case structures. Regarding coercion dots, I don't believe replacing the automatic coercion with an explicit coercion should make a difference in performance (although I'm not disputing that it does in your version of LabVIEW).
Regarding the selector instead of case structure, I would like to know whether the selector computes all inputs before evaluation or if it smartly evaluates the condition first. It may indeed be smart, but it is not obvious from a dataflow point-of-view. I therefore would err on the side of code clarity to use the case structure instead of the selector.
When I decided to add error handling and input validation, it was because I realized that I had performed premature optimization.