Make one big site with multisite

Hello, my problem is that :

I want to do a single site but with approximatively 300 modules on it. My site has to do a lot of sites.
With this amount of modules we can't forget performance (and I don't want).
So, my idea was that:
In place of a big site, i create a lot of small sites communicating between them.

My ideas are:
To create these sites as subdirectories of the main site to avoid the login problems.
Of course, to share the users tables among these sites.
To use the module front to make redirects to the login page of the main site.
To use the modules RestFul WebService and Rest Client to share entities between these sites.
To use the module UUID Redirect to make access between sites.
To use the module InsertFrame to display some elements between sites (I would like to use the module remoteBlocks but I am on Drupal 7).
To use the module ThemeKey for having the same theme among the sites.
To use the module Solr Multisite.
To admin with the modules Remote Dashboard, Production Monitor and Nagios.

Is it a good idea ?

How can we improve that system ?

The remaing problems are :
the Unification of the admin interface, the permissions, the translations, etc...

To share fields between sites.

Here a case study for the fields problem.

I have a site specialized in webform, one in OG, one in commerce, one in media and one in location. All these sites as backend of a main site.

I would like to have a webform containing a field belonging to each others sites and treated by them.

For example: a marketing video.

First field: the target group (treated by the OG site), an OG_field
Second field: the place (treated by the location site), a geocoder_field
Third field: the commercial link (treated by the commerce site), a commerce_product_field
Fourth field: the video (treated by the media site), a emfield

And this webform has to be displayed by the main site.

How is it possible ?

Any suggestions ?

I think, on a long term, a suite of modules doing that can be a good idea.