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

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.

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.

yes they will appear bigger without the compression from the muscle, but they will droop to your knees as the skin will not hold them up very well. you will become progressively more stretched out

I would be careful what you desire as far as implant size.  You have already had a breast lift, a larger implant, such as an overfilled 800 cc saline implant will only make the breasts heavier and lead to stretch deformities.  

Large
implants like yours may look fuller if placed over the muscle rather than
under, but then there just isn’t enough supportive tissue to ensure positive
results long-term. Submuscular placement is the more practical solution as far
as longevity is concerned for implants in any size, and this goes double for
larger implants. Although you may feel your figure looks curvier initially with
subglandular placement, the weight of the implants will stretch out your
natural breast tissue much more quickly, likely requiring additional surgeries
later on to maintain your look.