I always worry when someone suggests that you can use tabs on a control surface (trim,boost, servo, or otherwise) to trim a model. Full size yes, but a model no. (Apologies to Dick if this isn't what you mean). I remember reading of a large model Lancaster bomber crashing in the UK due to this. This was back in the early 80's when large models were relatively new and servos were relatively weak. The model was fitted with an adjustable tab on the elevator. The model needed nose up trim so they adjusted the tab trailing edge up. This pushed down on the trailing edge of the elevator, which was held immobile by the elevator servo, and therefore lifted the nose of the model. Problem solved right?

All was good until one day they flew the model a bit faster than previousely. The tab overpowered the servo and pushed the elevator down.... Splat! Up elevator was no use, the servo wasn't strong enough to overpower the tab. This occured at a model display IIRC, luckily the crash missed the spectators.

In a full size plane (with non powered controls) the pilot holds the controls where required (boost tabs will assist if fitted) and then the trim tab (could be the same tab) is adjusted to reduce the control force to zero.

In a model the servo holds the control surface where commanded, any trim tab is only going to load the servo adding stress to the linkage and using up battery power. Fly it fast enough and either the linkage or the tab or the control surface will break, or the servo will be overpowered.

I read an article where a french modeller mounted a fuselage to the roof of his car and measured the elevator deflection at up to 100 mph or so. Even though the servo was strong enough there was still a significant reduction in throw due to flexing of the linkage and control surface it self. A servo tab gave full throw.

Other aerodynamic balances work well on models eg horn balances (like on a cub rudder), inset hinging, spades etc. Even on aerobatic set ups at 45 degrees. Boost tabs also work in these situations although the linkage might get a bit difficult.

So the way I see it:

pros:

Reduces required power (weight and cost?) of batteries and servos
Improves speed and centreing of servos
In a scale model, if the original had them...

cons:

Fiddly and possibly not reliable
Don't achieve anything that a stronger servo wouldn't come close to achieving.

To th OP, it's your model, could be a cool feature.