I have super heavy flow - going by what you've said mine is actually heavier than yours. I've not used the Femmecup - have used Mooncup, Divacup, Lunette, Ladycup and MeLuna - I personally prefer my (large) Ladycup, it works fine for me.
What I would say is not to just go for larger capacity - I went for the largest sized MeLuna and found it was no good for me, it was too firm so I couldn't get the suction needed. Always figure out what would work best for your body and go from there on picking capacity - community.livejournal.com/menstrual_cups/1285963.html - also of course try asking this question on the Menstrual cup support community for more advice from other cup users.
You could always try the Femmecup (who are currently doing BOGOF deal FYI) and sell it on Menstrual Cup Sales -community.livejournal.com/mc_sales if not working out for you. Or hell I've got a large unused MeLuna spare you could try.
Obviously any cup you wear you'd still go longer than with a tampon, you can wear overnight which can't be done with tampons, and cups may lessen flow in comparison to tampons. Whatever it's still going to be better than tampons or pads.
If heavy flow is that much of a problem for you, how about lessening your flow? Tranexamic acid from your doctor is the best way to lessen flow. Home methods include drinking coffee (a vasoconstrictor so constricts blood vessels in the uterus preventing heavy blood loss), taking herbal supplements to regulate hormones (chaste tree tincture is best) and dietary changes or vitamins; iron, magnesium, zinc and calcium for menstrual health, vitamin E to regulate flow, kelp tablets to revitalize the endocrine system, and vitamins A and C to regulate estrogen levels.