I'm looking at getting an ARDcase set but was wondering does the topbox affect the handling.

It looks a bit of a monster!

Is 34l the best size of pannier in this set-up?

Ardcase Luggage
 

I have the 50 L case set up and I can only say it's a very durable and reliable set. I must've come off my bike at least 100 times and no substantial damage to the cases. Fully recommend these cases to anyone, price and durability is unmatchable And by the way he offers great customer service.
I do have a couple concerns but they do seem minor compared to all the stories from other riders who possess alternative cases.
Those being they are a little bit heavy and the Lids will fly unless you are careful with the locks which I am not. Four example when I had soft luggage, i used to permanently ride around with the zips open and they would fill up with water and the zips would break etc.
I think some sort of hinge or tie would be better