So your adding a third raid...  The raid subsystem will be mostly affected by how much all the combined raids arrays are utilized, and how much bandwidth of the system bus is utilized, CPU/memory speed. If your server raids are hammered, it will affect I/O overhead. if moderately/mildly utilized, where the bandwidth of the raid adapter/ system bus are not totally utilized, and the raid cache is not overloaded, with occasional spike in usage, you would be hard pressed to notice any performance drop, if there is any drop at all. If you go tweakers.net , (in Dutch, but you can get around without the language), they have benchmarks for all types of raids, and with multiple disk counts. From a few raid 10 setups I looked at it does not look like your proposed addition would saturate the system, I saw a few benchmarks with 12 drives which had performance gains over 8 and 10 drive arrays. Even though the benchmarks are for single array systems, you can judge how a multi array system will behave, aside from the fact benchmarks are placing a high load on the adapter/system.  There is no other site I have found with the degrees of raid benchmarks [URL]
My current configuration is:  Dell PowerEdge R510 with PERC H700 RAID controller. I have 2 x 300Gb 15k disks and 6 x 600Gb 15k disks. The 2 x 300Gb disks are on RAID1 for the OS The 6 x 600Gb disks are on RAID10 for the DATA If I add 4 x 600GB 15k disks on RAID10 would I see a performance drop in I/O on the other RAIDs? 
Performance degraded I/O on RAIDS