I'm not large-chested, or a swimmer, so this idea might be absurd...What if you wore the top to a two-piece suit (something like this?) Under your normal suit for extra support and coverage? Or even sacrifice a sports bra for the job?

Edit: on the speedo high neck you posted, the two negative reviews say that it's meant for someone busty on top and narrow on the bottom, so maybe it could work for you!