Another consideration is Web Reports. If you are going to have many (>20) simultaneous Web Reports users you should consider putting it on its own server.
The performance of BigFix/TEM/IEM is often limited more by Disk speed than anything else. The other major consideration is configuration and structure of your infrastructure.
I definitely recommend either SSDs in RAID1 or PCI-E SSDs with frequent backups for the DB and FillDB. See this:FillDB Buffer Size Issue
Are the 6 tier 1 relays running on server class hardware with fast NICs?
Are the 3000 tier 2 relays all on at least Gigabit? Are they user workstations?
How is the performance now? Is it satisfactory?
How many clients to you anticipate adding?
Are you using Message Level Encryption?
How many simultaneous console operators do you have / plan to have?
How many simultaneous web reports users do you have / plan to have?