BuddyPress.org

@noyzen,
If i follow correctly what you’re saying, a post of 1000 words, is good for SEO and that’s why you want an activity feed with over 300 char posts excerpts. But this is practically duplicate content, which is absolutely not good at all (seo or not seo) :-;
That said, the default post excerpt length is 250. And if it’s default, it’s probably also the default best practice value.

But you’re the boss…
Add this to fuctions.php or bp-custom.php to get your activity stream exerpts longer without duplicating it.

function bpfr_custom_length( $excerpt_length) {
	$excerpt_length = '200'; // change value to your need	
		return $excerpt_length;  

} 
add_filter( 'bp_activity_excerpt_length', 'bpfr_custom_length', 10, 1);