The most important part of ANY system is the data, and to a customer the most important data is THEIR data.

When I was showing geodemographic marketing systems we would deliberately populate the system with the customer's own data so the customer could relate to what they had been shown.

The brighter customers will use your system to validate a fact that they already know about their data.

A good sales pitch should be tailored to the customer.  You should research your customer to the point where you can predict 90% of the questions they are likely to ask.  In effect you "storyboard" the sales pitch.

Now I am working for a web development company using a number of CMS's we do demo's on high spec laptops with Windows 2000 Server installed.  In our case the fact that the system can run on perceived low power machinery is a plus because the IT types will see that they can use the system for development on the crap that their finance department expects them to perform miracles with.

Anyone who is not an IT guy thinks "Wow, look at my data"!  They will also have the attitude (unless your system really is a dog) that performance is an IT problem and that they are just interested in the functionality.

It is implicit that the live system won't be running on a laptop so part of the preparation for the demo is making damn sure that the demo is slick, works fast and well!

If you want to be honest about the performance of your system then publish benchmarks and ask former customers to act as references.

DO NOT do a demo where you imply "here is an example of our system crawling/crashing/trashing everything else on the server".  Your potential customer will remember the fact that it crawled/crashed/trashed everything else on the server and not the why's and wherefores.

Don't get me wrong.  Be honest but not too honest.  Accentuate the positive.