Generally in the mix, the smaller and less productive bird drags down the larger one genetically in the offspring, but the results are not always linear.
I have a bantam Cochin hen crossed with a Barnevelder rooster as I wanted to size up my brooding hens. Bantam Cochin Momma is very prone to brooding. I was expecting about a mid size.
However, that female chick is surpassing my expectations size wise. So it can vary as to the size you get. I've had other mixes that were more of a middle ground.
As to laying, it depends on the Silkie line. I've had Silkies that didn't lay well at all, even when they weren't constantly brooding, but I had one gal that almost kept up with my production layers.
And that's another factor you will be folding into your line...broodiness. Silkies are known for excessively brooding. Brooding is not laying, so it is likely your eggs will be smaller and your productivity will go down as the daughters want to brood a lot.
You will also be adding foot feathers. In rainy weather, that can be a magnet for bumble foot. The first generation will be less feather than the Silkie, but breed backs will regain his genetics, so each generation will get smaller, less productive, and foot feathered.
If you want to refresh the flock with productivity and sustainability, I personally would not use a Silkie rooster. I would choose something from the Wyandotte or Hamp (New Hampshire??? really good choice as they are generally nice boys) or even the Ameraucana/Easter Egger, though if not fully Ameraucana, you have mixed results as the EE is already a mixed breed.
I've done quite well with my Barnevelder boy. Each generation breedback has taken on his calm characteristics, and amazingly, by F1 (1st) and definitely by F2 I've had hens with really nice double lacing and nicely terra cotta eggs.
So...yes...choose your rooster carefully for the goals which you wish to promote over time.
LofMc