I'm 5'8, 160lbs with 800cc implants under-the-muscle. Will a revision with implants over-the-muscle, make them appear bigger?

I would not suggest converting your current breast implant position to the sub glandular position. There are simply too many problems associated with breast implants in this position, especially with larger breasts implants. I think that you will find the attached link, dedicated to larger breasts augmentation patients, helpful to you. Best wishes.

There
is another alternative to make your breast look bigger.Aligning the areola, breast tissue and
implant over the bony prominence of the chest wall maximizes anterior
projection.Unfortunately, you have not
provided any photographs but after 800 cc implants your breasts have
descended.Many women refuse lifts
because of the ugly vertical scars.That
is why a new technique was developed called The Mini Ultimate Breast LiftTM.Using only a circumareola incision it is
possible to reshape your breast tissue creating upper pole fullness, elevate
them higher on the chest wall and more medial to increase your cleavage.Best Wishes,Gary Horndeski, M.D.

Having had implants both above and below the muscle, you're probably aware of the difference of the look.  Another important difference is the loss of the muscular support when implants are above the muscle.  As a fitness instructor, you may be especially troubled with the resulting looseness.  Another issue is the incremental increase in the risk of complications with every operation.