I've found that a teleconverter does not add magnification but it increases the focal length and working distance. The mimimum image size is the same but you can achieve it from farther away. A 1.4 teleconverter reduces the light by one stop. A 2x reduces the light by two stops. Teleconverters generally don't work well on slow lenses but do work on f/2 and f/2.8 lenses. They reduce image quality, but not significantly on the faster lenses.
You have plenty of light with dual strobes so small apertures with a TC at macro distances are not a problem. No one wants to shoot macro wide open because of the negative affect on DOF. It just gets harder to autofocus when the minimum f-stop drops to around f/5.6 or f/8. Otherwise reducing the aperture is really not an issue for UW macro.
I'm shooting macro with the Olympus E-330 in an Ikelite housing and the 50 mm f/2 macro lens (100 mm equivalent). It provides 2:1 macro which is a 34 by 26 mm minimum image size. On Olympus 4/3 format, using the 35 mm f/3.5 macro lens allows 1:1, or an 18 mm by 13 mm subject size. That's a really small image size. For topside wildflowers and other macro I use the the E-5 with the 50 mm lens and a flash ring.
The 50 mm macro achieves 2:1 at a reasonable working distance, several inches away. The 35 mm macro lens achieves 1:1 at about 1" from the lens, or right at the lens port face. That's not useful underwater, but you can still focus the 35 mm macro at less than 1:1 magnification from farther away. The 50 mm is a better lens for UW macro on Olympus 4/3.
The 2:1 macro image size on 4/3 is essentially equal to what the full frame camera achieves at 1:1. The 4/3 sensor is only a little bit smaller than the APS-C sensor size. Apart from recent advances with newer APS models in high ISO and DR, the IQ and functionality is similar. For macro UW with strobes shooting at low ISO is typical so the differences are marginal.
An extension tube increases magnification without reducing the light. With the 50 mm f/2 macro and the EX-25, a 25 mm extension tube, it achieves 1:1 from a closer distance. In-focus range is limited to between 2" and 4" with the lens focused from minimum distance to infinity. I've used this combo a lot topside with a ring flash and the image quality is great. The subject size is as small as 18 mm by 13 mm with the Olympus 4/3 format.
For UW macro here are the downsides to using this 50 mm + EX-25 combo (and probably with any system): It's very hard to find subjects within the focus plane at that magnification. Everything is a blur until you bring the subject right into the focus distance. Autofocus is awful unless you're already close to correct focus. If you're too far away for the lens + EX it will never achieve focus. Shooting topside with the EX-25 I typically manual focus at anywhere from minimum to infinity and then move in until I achieve focus. It may be possible to arrange the Ikelite zoom ring to function as a focus ring even with the EX-25 in place. I put the zoom ring on the focus ring for both the 50 mm macro and the 8 mm fish eye lens since there is no zoom and it works well but you can not see the focus scale. For UW behind a flat port the 50 mm + EX may be far enough away At the available focus distances, but strobe lighting will be difficult.
I've found the Olympus 50 mm f/2 macro lens to be very capapble for UW macro. I haven't had any desire to increase focal length with the EC-14 because the 50 mm is far enough for most subjects. It's also challenging enough to aim and focus with just the lens so I'm not ready to make it more diffficult with a longer focal length.
As other people have said, I think achieving greater magnification than 1:1 underwater is probably better with a longer focal length. I may try this using an extension tube with the Sigma 105 mm or Sigma 150 mm f/2.8 macro lenses.
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