Yes, it works.
Just elaborating the steps per James at #11 and Shane at #25 above.
Plesk Control Panel
1. Add PHP5.5 (or another) through the Plesk control panel and verify.
SSH into the Box
2. Login to the box with sudo/root access.
3. Create a custom folder in /usr/local/psa/admin/conf/templates/ if it had not already existed.
4. Copy /usr/local/psa/admin/conf/templates/default/roundcube.php to /usr/local/psa/admin/conf/templates/custom/.
5. Edit /usr/local/psa/admin/conf/templates/custom/roundcube.php to change 2 entries:
PP_CUSTOM_PHP_CGI_INDEX fastcgi ---> PP_CUSTOM_PHP_CGI_INDEX plesk-php55-fastcgi
6. Run php -l /usr/local/psa/admin/conf/templates/custom/roundcube.php to catch any syntax errors.
7. Run /usr/local/psa/admin/bin/httpdmng --reconfigure-all
8. Check /etc/httpd/conf/plesk.conf.d/roundcube.conf to verify that PP_CUSTOM_PHP_CGI_INDEX is now pointing to plesk-php55-fastcgi.
Cheers.