It very much depends on the application and the server you were previously using.
The memory consumption for the application should be similar, but of course the memory consumption of the server compared to dm Server is likely to be different.
In terms of CPU and response time, I would expect dm Server to be similar to Tomcat as the request processing path is similar.
Performance predictions are best followed up with detailed measurements. Please post back your detailed findings if you would like to.
Glyn Normington
SpringSource