What are the benefits to group mating vs individual mating like you say with your leghorns. Next year will when you go to four pens how will the mating be then? Father/daughter, mother/son.?
The reason I'm going to clan style breeding set ups (I've used them before), is I don't really have a desire to have to bring in outside stock, and in some cases it's hard to find (like with my Leghorns and Langshans). The advantage is you can go almost indefinitely without having to bring in new blood. It also really tightens up the line, after a few years all the birds start looking like clones of each other, at that point can usually get away with hatching less, which means either more to sell, or more time/space for other projects.
Edit: One downside to this method...you can NEVER double up faults. I'll use Leghorn as an example, if I have a male with too long of a back, he must NEVER be bred to a female with too long a back. When using a system like this if you double up faults you will have an almost impossible time breeding away from them once they get set. If you end up with this happening, you need to either switch to a back-up breeding bird that doesn't create this issue, or not breeding from that pen that year.
To break out to 4 pens next year I will do something like this (2014 Pen C is the eggs I'm buying from local breeder, keep in mind all chicks hatched are marked with pen they were born in):
2014:
Pen A - Pen A Cockerel, Pen A Pullets
Pen B - Pen B Cockerel, Pen B Pullets
Pen C - Pen C Cockerels, Pen C Pullets
2015:
Pen A - Pen C Cockerel (From 2016 on labeled as Pen D Cock/Cockerel), Pen A Hens/Pullets
Pen B - Pen A Cock/Cockerel, Pen B Hens/Pullets
Pen C - Pen B Cock/Cockerel, Pen C Pullets
Pen D - Pen C Cockerel, Pen C Hens (From 2016 on called Pen D Hens/Pullets)
Then every year will be rotating males one pen down the line. Every year I adjust the number of females in the pens based on my need, and expected demand to sell as well. For instance if I don't plan on hatching quite as many birds in 2015 I might not need the Pen A Hens, but I won't get rid of them, and may use them in 2016 if needed. Nine times out of ten, once a bird makes it through all the cuts and culling process to actually make it in the breeding pen that bird is going to retire and stay on the place til it dies of old age, being shown and/or providing eggs as long as it lives. This is opposed to show birds that make all the cuts except the breeding selection cuts, they'll be shown and then sold off.