Yeah thats not great. Generally if I get 20,000 or above I see that as acceptable for our window. I'd say on average we get about 40,000 kb/ps over SAN transport but do get a lot of variance. The range is from 20,000 all the way up to 130,000 kb/ps. If the I/O streams at the max say 20k - 50k. If less going on we hit some of the higher figures.
We finally got rid of tape after moving our 2nd 5220 to our second data centre and use bi directional AIR to effecively offsite our backups. I've found tape duplication certianly has a negative impact due to it have to rehydrate the data and pull the various blocks from all over the disk (this is where the rebase comes to play to minimise this as best it can). As AIR is only the unique blocks the read hit is much less.
In terms of the VC database, we found it was sharing a LUN with a couple of other SQL servers and just wasn't getting the IOPs that it needed. We are running on an EMC CX4-480 which doesn't have the best reporting for IOPs so had to dig around to pull the stats (soon going to VNX which should be much better). When the backups were in flight we found a marked decrease in Vcentre performance which had a knock on effect for backup performance as well as reliability. We've actually given the server a dedicated LUN now and it much improved our situation.
Best bet is to check out the data path all the way from source to target. Get your SAN stats (not just from vmware but from the actual array) to see if its touching the sides then work back to your SAN swiches (port errors, faulty fibre etc).. I'm of course assuming you're using SAN and not nbd. If your using nbd I think it'll be using the ESX host management network for the backup so maybe check that out too.
My problem was that we are a windows house and going to the 5220 on Linux was a bit of a step into the unknown and I felt pretty helpless when trying to troubleshoot potential non netbackup issues on the applliance (for example disk utilisation, memory etc etc). My advise is to do all you can to rule out external factors in your environment. Symantec support can be extremely hard work but speak to your account manager. They are really kean to prove how great their appliances are and I'd be very surprised if they wouldn't send an appliance engineer to your site for the day to assist you if you express your disatifsfaction (they did with us and were prepared to subsequently send a "technical expert" guy until we got things running well).
Hope this helps.
Ed