I've been doing some tests, using Apache HTTP 2.2 as a caching proxy in front of DotNetNuke 7.2.2. Even with module caching etc in place, the response time for a page goes down from a few hunderd millieseconds to a few milliseconds when you have the page cached in the Apache daemon. However, since for some unknown reason even public pages have "Cache-Control: private" you need to add some extra settings to your Apache configuration:

CacheEnable disk /
CacheRoot /some/path/to/cache/dir
CacheDirLevels 3
CacheDirLength 5
CacheIgnoreNoLastMod on
CacheStorePrivate on
CacheStoreNoStore on
CacheIgnoreCacheControl on
CacheIgnoreQueryString off
CacheDefaultExpire 86400
CacheMaxFileSize 100000
CacheMaxExpire 172800

Of course, in this case it becomes quite hard to log on since you will always get served from cache when the page was ever requested before. And you should make sure the cache is emptied after fetching some pages when authenticated.

So, I am curious for the results of your implementation on basis of OutputCachingProvider.

That's good but all this is about client side caching (correct me if I'm wrong). You can set this in web config too and have different settings for different folders too.
The "client" part of the DNN cache is something I didn't look at yet.

The OutputCache is server side caching, to avoid all the page life-cycle and eventual sql queries