Any GB owners out there replaced their swim platforms with an alternate material, i.e.. non teak?
You might try posing your question to the Grand Banks Owners forum if you haven't already. (I haven't seen your question there but I don't look at the forum every day.) The URL is
http://www.grandbanksowners.com/ You need to join to post but it's free.
I've seen a few GBs in the yard over the years and at the dealer dock with non-teak swimsteps. As I recall, they were solid platforms with freeing slots cut/molded in them. I have always assumed these platforms were fiberglass.
I cannot think of any reason a "normal" slatted platform couldn't be made of Starboard or some other synthetic material as long as it was strong enough for the job and very UV-resistant.
If done in an aesthetic and professional manner, I don't think a white solid or slatted swimstep would detract from the look of the boat. The ones I've seen didn't, in my opinion.
One thing to be aware of is the length of the support brackets. The stock swimstep, like the one on our GB, is not that wide (fore and aft). American Marine/Grand Banks offered an optional extended swimstep that is about twice as wide (if not more) as the stock step. However, the extended swimstep requires different brackets.
There have been numerous discussions over the years on the GB owners forum about extending the stock step. The accepted method is to bolt longer, silicone-bronze support bars on top of the stock bronze support bars, and then build or buy a new, wider teak platform to mount on them.
But..... there is a limit to how long these bars can be. IIRC, the collective wisdom of the experienced shipwright/former yard owners on the forum says that the stock brackets should be extended by no more than half their original width using the longer-bar-on-top method.
Joining the GB forum will let you search its archives for the discussions on this topic.