If you don't mind doing a little work, then you can get great NFS speeds and still get ACL by using Kerberos Authentication. Okay so the NFS export would be access to all, but then the MS best practice is to make all shares Everyone/Full and set the permissions using NTFS ACL's which are much stronger and more auditable.
But going back to CIFS, I believe that 10.5.x has a proper CIFS client that is built into the OS, where-as previous versions either still used SAMBA, or the code was a module based on this and not properly part of the OS. For this reason, CIFS works at it's best with 10.5.x.
Your original question of performance, it really does depend on a lot of factors. If that is 10TB of Quark files, then there may not be a huge issue, but if it's 10TB of Word and Excel, I wouldn't be surprised if it didn't run as quick as you might expect, and things may slow down the more files (as in, millions of files) you put on the share.
However, as far as processing power goes, the 2050 does give the 3020 a good run for it's money! If you are pushing a 2050, then you are not far off pushing a 3020 (depending on the work load). I'd imagine the key here is what disks are you using? Are you loading the system up with SAS and then extra 15k FC disks? If you are using 1TB SATA, then I would say you'll hit performance problems pretty quick with VMware on there as well.