First, you assume that it's not fun for some people to feel powerful and blast through certain enemies, especially in such a setting as an alliance raid. Second, if it's fun, then you wouldn't have problem convincing people. The poll means nothing since you said it yourself that you can't reliably get people to lower their item level to make the encounter harder.
The other solution would be to do a daily PF for alliance raid if you want like-minded individuals who want a fun, challenging experience in alliance raid.