For Silvia and really any machine that has a PID controller (whether being used as a PID or otherwise), the true value of measured temps can change "a touch" when using a thermocouple when the measuring instrument gets subjected to varying ambients. All of this would be "extra" to the fact that electronics and heat are not the best of friends.

When the temperature sensor is an RTD, this pretty much takes varying ambients out of the picture. All this said, mounting the controller next to the grouphead is OK - all things considered. Silvia will likely NOT be your last espresso machine and so resale of a "stock" Silvia or one that can be easily reconverted to stock is always a good bet.