How to do load testing for a web application built using JSON restful web service at server side?

this question is not about which tools to use to do load testing ...

It is a more general question about what should be tested.

background:

years ago, most enterprise web application used to be designed that front end and backend are messed together, and any kinds of business logic is done at server side. while front end's responsibility is only to show the UI.

when load testing such application, whether it is Oracle EBS or SAP's things, or application built using framework like Struts.

What we do is to:
1. design a user test case. (like login, search , put sth to shopping car, then check log)
2. write testing script to archive such test case (underline, it's pure http call to server side )
3. using LT tool to generate lots of virtual user together to run such test. (it will just behave like that there are lots of user do the same business logic together)

While now , the whole thing changed after after front end and backend totally separated.

Lot of business logic can be put in front end, and server end only has separate restful web service...

What i found is that we can't do load testing like before.

Just because user's test case can't be mapped to http communication. As some task is totally done in front end...

BUT in such new world, how to do performance testing? Just performance test the JSON restful web service? How about the static resource transferred from service side to browser ?

I need suggestions from these who done lot of performance/load testing for such JSON restful web service application..