Upgrading/mixing drive sizes

[Asheldrake]

Hello, this is my first post, and I dont know much about all this. I was hoping someone could offer their input on a problem we are trying to solve in upgrading our drives on our Promise VTrak e610f/j610s system?

Currently we have two head units With dual controllers attached to a single jbod expansion. Each is populated with 300gb SAS drives, and is split up into the following LUNs:

Chassis Head 1 and Expansion 1
2 x Hot Spares
1 x 2-drive Meta data
4 x 7-drive Raid5

Chassis Head 2 and Expansion 2
4 x 8-drive Raid6

These all have affinities to two video storage pools and are striped into a single volume managed by xsan.

We need to expand our volume and we intend to do so with 2tb sata drives. We can however at the moment only afford 25 drives... How would you reccommend using them in a way which expands our capacity and includes still using our old drives, while maintaining acceptable bandwidth.

It was our thought to replace three of the 7-drive raid5 luns and the hot spares and keep two spare out of the chassis. But someone expressed concern over balancing the system and mentioned that we will eventally see problems occuring.

Because of this, i decided to split the new drives accross the system and wondered if this would balance the load appropriately. My only question about my plan is whether there is a downside to using arrays with less drives... Here is my revised plan:

Head Unit (+Jbod) 1 will have the following:
1 x 2-drive meta data (300gb) - (600GB)
2 x 5-drive Raid5 (2TB) - (16TB)
3 x 6-drive Raid5 (300GB) - (4.5TB)
2 x Hot Spares (2TB)
 
Head Unit (+Jbod) 2 will have the following:
2 x 6-drive Raid6 (2TB) - (16TB)
3 x 6-drive Raid5 (300GB) - (4.5TB)
2 x Hot Spares (300GB)

I wasnt sure if there are downsides to using less drives in the arrays. If there are any, please someone let me know! Either way, suggestions for my situation would be most appreciated!