The HP MSA1500CS is a pretty wimpy device. I have one, and I hate it. I'm somewhat surprised it has kept up with your stated workloads. It probably comes as no surprise that I recommend upgrading to the MSA2000. It has a much better storage architecture than the 1500CS, and can scale better.
Without more data I can't recommend going to an EVA4400 (HP's 'entry level enterprise array') versus the MSA2000. The 4400 will take you a lot farther than the MSA2000 will in terms of scale out, but I don't know what kind of growth you expect.
RE: LeftHand vs. MSA2000
So long as you have the ethernet network for it, the LeftHand unit should out-scale the MSA2000 by a long shot. The distributed storage controller it uses makes that kind of thing easy. You'll pay more per storage shelf, but you can scale to silly amounts with it. Once you start hitting the I/O ceilings on an MSA2000 (which will depend on the drive technology you use as well as any active/active configs you can use) you're pretty much done. For the LeftHand products that ceiling is a lot more mushy.
Where the LeftHand approach really saves you is with parity RAID. Doing rebuilds after a failure is the most CPU intensive thing it does, and is where my MSA1500cs falls flat on its ass. On my 1500cs, rebuilding a RAID6 array across 6.5TB of disk took about a week, during which time it was deeply intolerant of large scale I/O writes to anything on the array. Since LeftHand has a controller in each cabinet, restriping a LUN on one shelf will not affect performance of LUNs on other shelves. This is very nice!
All in all, if you have the budget for it the LeftHand devices should serve you a lot longer than the MSA2000.
Answering my own question here in the hopes it will help others. We did this today and I have to say the pucker factor was pretty high:) Changing the first SAN to jumbo had no problems at all. When we broke the MPIO on the second SAN all the hosts got very angry (stopped responding) and it took about a minute for them to recover. Our Office Communications Server 2007R2 refused to play ball and required a reboot. Other than that it worked but I'm thinking next time (never will happen now that I know to configure jumbo before going live) I'd rather do the hosts last because then we just worry about a single host at a time.
Best Answer
It can be used as a stand-alone, but you set yourself up for failure by doing so. The Lefthand solution is like a networked RAID configuration. If you purchase just a single node, and it fails (as sometimes happens with any server), you are down. Additionally, there's a severe limit to the number of spindles they can handle, so performance bottlenecks very quickly in a high-throughput application - you wouldn't want to put any SQL servers on here for instance.
A really important question here is, what are your intentions for a SAN? The correct solution very much depends on your intended application...