Hi Derek
I put together a bench with 100mm x 100mm legs a while back and I think it was overkill, so you should have no problem with your choices.
Are you planning on putting the leg tenons right through to the top of the benchtop or only half way?. I'm planning on doing the bench from Mr Schwatz's first book but made of pine laminations and was thinking that by putting them through into the top I could build the mortices into it and then glue and screw them into it before adding the outside rails, which should hide the joins.
How tall are you?. I'd suggest that 86cm is a big high for someone who uses hand planes as much as you do
Also, why square dog holes? Wouldn't round dogs give you better flexibilty if you want to work anything that's not square?
Cheers
Andrew