PowerPress doesn't handle the post content mapping to the feeds. What you enter into a blog post content field and excerpt are mapped to your feed directly by WordPress, whether or not you are using PowerPress.
You may want to search WordPress.org for a plugin that lets you modify the RSS2 <description> and <content:encoded>.
So you are aware, the <description> RSS2 tag is populated by default from your blog post content, stripped of the HTML. <content:encoded> takes your blog post content as is, including the HTML. There is an option in WordPress to use the excerpt in place of the <description>, but I am unaware if it also applies to the <content:encoded>. Many feed readers such as Google reader will default to using the <content:encoded> over <description>. To get the exact desired effect you are looking for, you may be a plugin that simply removes the <content:encoded> from your feed. That combined with using excerpt for <description> will do exactly what you want.
If there is no plugin out there to do this, you may need to follow the instructions in this article:
http://www.memwg.com/enabling-wordpress-full-feeds/ This would require you to modify WordPress directly, and every time you update WordPress you would have to re-update your wp-rss2.php.