What would the performance implications be if we consolidated the customers into a single DB? 

Only you can answer this question. Not knowing your environment or your server configurations this would be difficult to predict. There are tools out there that can be used to stress test your current configurations to see what the limit is they can handle.

You should setup a script to gather the performance counters over a period of time so you have a good baseline of your systems. You will be able to use this to find your peak times of traffic for each one. You can do a quick Google search for "SQL Server performance counters" that can point you in the right direction. I am also sure stackexchange has many post on it as well.

EDIT This would be a good link to start with for the performance counters.

What would the performance implications be if we consolidated the customers into a single DB?

Only you can answer this question. Not knowing your environment or your server configurations this would be difficult to predict. There are tools out there that can be used to stress test your current configurations to see what the limit is they can handle.

You should setup a script to gather the performance counters over a period of time so you have a good baseline of your systems. You will be able to use this to find your peak times of traffic for each one. You can do a quick Google search for "SQL Server performance counters" that can point you in the right direction. I am also sure stackexchange has many post on it as well.

EDIT This would be a good link to start with for the performance counters.