How to sync with plugins update after I have done manual optimizations?

I have several WordPress running plugins on my site and as per suggestions of wordpress performance optimizations I am planning to do the optimizations on plugins myself.

However, that will mean that I will have to modify existing plugins codes. Now, as WordPress plugins release frequent updates, the updates will wipe out my manual changes. How can I keep sync between these? That means, how can I get updated plugin and keep my manual changes as well?

Create a local repository in Git (or SVN) for the plugin, and each time an update happens:

• merge the changes into your adjusted version,
• test it in your local copy of the production site,
• then push your updated code to your site.

But much better would it be to send your improvements to the plugin author, so she can use that in the main code. Keeping improvements secret is not the spirit of Open Source.