I did a little project on this a while ago, WR vs Number of tables. If I can dig up the link, I'll post it. But basically, yes, your win rate in bb/100 goes down with each successive table, but your wr in bb/hr goes up. There does come a point where the addition of another table has a larger negative bb/100 affect than positive bb/hr. You need to try and find that point where you are maximizing your bb/hr, not neccesarily your bb/100. The following is a graph of this effect.
A interesting little corollary I found while working on this, is that due to a reduction in our Standard deviation, in particular our sd/hr, while possibly having an increase in our sd/100, our minimum required bankroll (RBR) goes down. The common accepted value of FRLH bankrolls is about 300 bb's for a safe BR. With each additonal table, our SD drops, reducing our potential to go broke. The reduction in RBR is so severe, that there comes a point where it drops lower than the minimum total normal (c 30 bets) buy-in for all the tables you have open. In my heyday, When I was 8 tabling, my RBR was around 160 bb's, which was the same risk of ruin % as 1 tabling with a RBR of 245. Even though the math dictated I only needed to have 160 bets to 8 table, I needed 240 bets to just physically sit at them. And just like the WR, there is an inflection point within this system, where our bb/hr drops, our sd/hr rises, and our RBR grows exponentially. *Just a caveat here, This last bit is based on the math figured I was aware of last year. I am now Delving into Chen's math book, and I will see if this theory holds water then, but so far my numbers have show it to be true.

"Rooting for the Yankees is like rooting for the house in blackjack." -Adam Morrow
I swear to God, next time I have to come back here, I'm bringing a stun gun and a weedwacker!