How to re-architect a portal for creating mobile app
What you guys think about architecture?

This is a common Service Oriented Architecture with decoupled Clients. You just have REST endpoints on your backend, and any Client can consume those endpoints.
You should also think about:

Do you need RESTful service (RESTful == stateless, will you store any state on the server?)
How to scale the service in the future? (this is a legit thing as you already aware of huge traffic increase and assume 2 servers)


How it can be improved?

Use scala instead of python :)

Will performance of portal will go down after adding above layers in architecture?

It depends. 
It will get some performance penalty (any additional abtract layer has it's tax), but most probably you won't event notice it. But still, you should measure it using some stress tests.

In the above architecture whether 2 servers should be used to run this (like one for client and other for serving the API's) or one server will be enough. Currently Heroku is used for deployment.

Well, as usual, it depends.
It depends on the usage profile you have right now and on the resources available. 
If you are interested in whether the new design will perform better than the old one? - there are a number of parameters.
Resume
This is a good overall approach for the system with different clients.
It will allow you:

Totally decouple mobile app and frontend development from backend development. (It could be different independent teams, outsourceable)
Standardize your API layer (as all clients will consume the same endpoints)
Make you service scalable easier (this includes the separate webserver for static assets and many more).
Currently I am working on a portal which is exposed to end users. This portal is developed using Python 2.7, Django 1.6 and MySQL.
Now we want to expose this portal as a mobile app. But current design does not support that as  templates, views and database are tightly coupled with each other. So we decided to re-architect the whole portal. After some research I found following:

Client side: AngularJS for all client side operations like show data and get data using ajax.
Server side: Rest API exposed to AngularJS. This Rest API can be developed using either Tastypie or Django Rest Framework (still not decided). Rest API will be exposed over Django.

I have few questions:

What you guys think about architecture? Is this is a good or bad design? How it can be improved?
Will performance of portal will go down after adding above layers in architecture?
In the above architecture whether 2 servers should be used to run this (like one for client and other for serving the API's) or one server will be enough. Currently Heroku is used for deployment.

Currently portal is getting 10K hits in a day and it is expected to go to 100K a day in 6 months. Will be happy to provide more information if needed.