The original plan for the project is.. it will only be a webapp. I decided to start playing around with grails.
I wonder if grails as our backend for cross platform mobile app is recommended (based on actual experience )?
I consider scalability and performance of the project more than faster development.
Yes, Grails is perfectly acceptable for enterprise level backend framework. There is nothing in Grails that prevents this. The only thing that will keep you from achieving scaleability and performance is you (and your own code).
There are lots of big companies using Grails in such a manner. Netflix comes to mind plus the Success Stories on the Grails website.
Given that Grails is a Spring application at heart you can always get back to just pure Spring and Java if you need, but as I stated earlier you will likely find your code is the bottleneck before the framework itself becomes an issue.
Proper design and architecture plays a critical role, but that holds true for any technology stack.