Jetpack Infinite scroll conflicting with theme’s pre_get_posts custom posts_per_page

I have the current function on my theme using pre_get_posts, currently right at the bottom of my theme’s functions.php file:

function mytheme_portfolio_archive_pages( $query ) {
    if ( is_admin() || ! $query->is_main_query() )
        return;

    if ( is_post_type_archive( 'mytheme_portfolio' ) ) {
        // Display 20 posts for a custom post type called 'mytheme_portfolio'
        $query->set( 'posts_per_page', 20 );
        return;
    }
}
add_action( 'pre_get_posts', 'mytheme_portfolio_archive_pages', 1 );

It means I have 20 posts per page if the current archive is of the post type mytheme_portfolio, instead of the default 5 for the blog pages (the number 5 is being set in “Blog pages show at most __ posts” in Settings → Reading).

The above function is working fine when Jetpack Infinite Scroll is deactivated.

Problem is I want to use Jetpack Infinite Scroll with the ‘mytheme_portfolio’ archive. And with it activated, the above function stops working and the Infinite Scroll uses the “Blog pages show at most 5 posts” number from Settings → Reading when it loads new posts. The Infinite Scroll is otherwise working fine but I need to show 20 posts per page, not 5, if I’m on a mytheme_portfolio archive page!

My Jetpack setup code is as follows, just for reference:

    function mytheme_render_infinite_scroll() {
        while ( have_posts() ) : the_post();
                if ('mytheme_portfolio' == get_post_type() && !is_search()) :
                    get_template_part( 'content', 'archive-portfolio' );
                elseif ( is_search() ) :
                    get_template_part('content','search');
                else :
                    get_template_part( 'content', get_post_format() );
                endif;
        endwhile;
    }

    function mytheme_jetpack_setup() {
        add_theme_support( 'infinite-scroll', array(
            'container'      => 'content',
            'footer_widgets' => 'footer-widgets',
            'type'           => 'click',
            'render'         => 'mytheme_render_infinite_scroll',
            'wrapper'        => true,
            'posts_per_page' => false
        ) );
    }
    add_action( 'after_setup_theme', 'mytheme_jetpack_setup' );

Thanks so much for any help anyone can give!

EDIT: I found the line in the Jetpack plugin code that’s conflicting, in jetpack/modules/infinite-scroll/infinity.php, line 26…

function __construct() {
    add_action( 'pre_get_posts',                  array( $this, 'posts_per_page_query' ) );
    ...

which corresponds to lines 449 – 452 in the same file:

function posts_per_page_query( $query ) {
    if ( ! is_admin() && self::archive_supports_infinity() && $query->is_main_query() )
        $query->set( 'posts_per_page', self::get_settings()->posts_per_page );
}

If I comment line 26 out my function works. Obviously I don’t want to go changing the plugin’s files, is there a way I can make sure my theme’s posts_per_page argument (initial code block above) is the one that’s used, and not Jetpack’s?