I have been shooting 2016's at 29.5" with a 125 gr Magnus 2 blade.  I wanted to see what difference I would get with a 200gr broadhead so I bought some and 6 200 gr field points.  

I'm shooting a Crow Creek Blackfeather at 51 lbs at 28" and my draw is 28" as well.  

I'd like to stick with aluminum since that is what I have always used anyway.

Will the 2016's be too weak?  If so what size should I try?

That's a question that is difficult to answer, it depends on a couple of factors. How the shelf of your bow is cut, less than center, center or past center. and where your 2016 falls within your usable spine range, if its close to being too stiff they may be fine, if they are close too being too weak they they won't work. If you already have 200 gr points the easiest way too find out is try them.
Bob

I would say they would be a little weak.  I shoot 2018 with a 125 gr. head and experimented with 145 gr. field tips with bare shat and found them weak just going up 20 grains. I might shorten them some more (room to spare on the length).I would thing a 75 grain leap would certainly affect the flight, especially with the broadhead. But I am still new at this arrow tuning and struggling a little with some carbons, so take it for what it is worth.

I'm thinking 2117...but that's just guessing off my experience with my setup(Which is very close to yours)

Do what I did and tune your bow to your arrows, if you like your arrow set-up. If they show signs of being too weak, then add material under your strike plate to cause your bow to be less than center-cut, which will cause your arrows to act as if they are stiffer. I use 28" 2016s with .257 caliber, 87 gr. bullets hot glued behind the insert plus 125 gr. points plus the weight of the insert and hotmelt. Makes about 240 grains total up front. I use wraps which also helps make them a tad stiffer than without. I'm shooting about 55#@26.5" with my Predator recurve, so I had to really build my strike plate out so I could use my 2016s. Glad I did though, as now I have about 19% FOC and a relatively heavy arrow to boot. And my bare shafts impact right with my fletched arrows.            

Going from 125 gr to 200 gr, you will need to do something.  You could shorten the the shafts an inch and that would help a lot as will building out the side plate.  Between the two, you should be able to get there.  You could also try a stiffer shaft.  Check out the dynamic spine calculator in Wingnut's post.  It will help you a bunch.

You could go to a 2018 or just build your side plate out a hair and use the 2016s you are shooting now.It will take very little change in centershot to allow you to go up 75gns on the point of the arrows you use now. jmo