Ouch.... does that mean, that you don't have any backup of your server at all? Isn't there any other server that could export a filesystem to your server to write an mksysb onto?

Ouch again. Don't get me wrong - I am a big fan of the "never change a running system" idea but running 5.2 ML1 has disadvantages for reasons of security and (probably more interesting in your case) in virtual memory handling. With ML1 you don't have the option to let LRUD sort out how to divide the RAM between program cache and data cache.

Looks reasonable for your configuration. You could increase the number of numfsbufs if you can afford to give more memory to it but you would need to unmount/mount the FS to take this change get into effect.

Maybe I can give you some hints which arguments to bring forward.
Is the server's response time meeting the SLA? The guys in charge are often completely ignorant to technical details but react very quickly once a breach of a contract is imminent or as taken place already. On the other hand if your boss says "so what?" (preferably in an email) lean back and relax. Everything is fine then. (Don't forget to tell them, that you cannot restore a backup easily. )
Technically: if this banking application is transaction based: how many transactions per second do take place? From that number you might be able to calculate how many disks your app should use (rule of thumb: a disk with 10Krpm handles about 125 IO per second (tps in iostat).) Organising disks in RAID can influence the number of possible IOPS. Use filemon to find out the applications disk's response time. Writes should not take longer than 5ms, reads no longer than 10ms.
If this banking application is database based: Can old data be archived to keep the DB indexes small? Are data and log filesystems on separate disks?