I have finished a major part of my game giving values for content in config.lua as 480 x 854.

I recently read in some article that 320 x 480 is optimum and using anything more than that uses more texture memory.

So I have to change everything relative to 320x480.

My question is : Is this absolutely necessary(for better performance) or can I just carry on with my game?

Where did you read this? I am not aware of this. I think you can carry on.
Texture memory is primarily dependent on the actual image files you load for your app, not the resolution your app runs at.

Here is that article!

http://blog.anscamobile.com/2010/11/content-scaling-made-easy/

The article says:
The thing to keep in mind is that texture memory (which is used to store graphics) is the most valuable and limited resource on mobile devices, and higher-resolution graphics will consume more of it.
and
It is generally not a good idea to use the iPhone 4 screen (640 x 960) as your content size and then scale everything down for the iPhone 3/3GS, because the higher resolution will quadruple the texture memory consumed.