I disagree with a few points on your post.

First off, I feel the power is adequate.  My gunners were around L103 when my BO was at 99, and I felt that while the spread could vary at times it still allowed me to deal out ~50k salvos.  I feel 50k is a good salvo for a L99 BB.  I personally don't consider 45 degrees a disadvantage in that it can penetrate armor more easily at max range, and it adds hangtime for chases.  While I prefer an aggressive ship generally speaking, I am still comfortable when I have to play patiently; this ship did not provide me with any trouble for the four levels I used it.

An example I can use to illustrate my point is that the Iowa is a strong ship and it is the same concept as the Missouri.

Secondly, I think the speed is not an advantage as you implied.  39 knots is somewhat average in comparison to other BBs (perhaps fast for the EBB5s, but average in general).  It is not easy to chase other ships down at 39 knots.

The key to this ship is to play it patiently.  It can pack a punch if you plan a good salvo and wait for the right moment.  However, with all of that being said I would still play the Washington any day of the week.

My gunners are 120 but the spread is so random at times, its very unlikely to land more than three shells on a rushing target.

I feel they shoul have made this ship like the Shinano, using the 18" duals and increasing the damage output and improving their spread.

I have one somewhere but still didn't test it.

Ya, poor range + 45 degrees + low damage, it doesn't look like a good EBB5 at all. 

45 degrees guns are good at dealing with AW's, Ill give you that, however it doesnt necessarily mean that they are good for defensive playing. Good luck trying to kill a 4 gold bar RM BB3 rushing you and waving at 44 knots with Missouri's spread.

The point I was trying to make is that the Montana gunset doesnt work well on the Missouri because the two ships dont share the same traits/strenghts/weaknesses. If you look at the L2/Temeraire comparison they are pretty much the same ship, three turrets, same guns, same damage output, same range.

Now if you compare the Montana and the Missouri you will notice they are very different ships and require to be played differently, so taking the BB5 gunset and slapping it on the Missouri doesnt make much sense.

edit: Im not saying the ship sucks, but its completely outclassed by other EBB5's.

Why so much about the 45 degree guns? The US kisses low angle guns goodbye after BB2, and 2 other nations (IJN and SN) are the same. If you are playing one of these nations, 45 degree guns is a given. If you want low angles, only KM and RM maintain them past BB4. Indeed, including event ships, there are currently more BB4+ with 45 degree guns than any 2 other angles combined (12 45 degree, 6 30 degree, 5 40 degree, 2 each of 34, 35, & 37), including almost universally praised vessels like the Stalingrad and Yamato. And, of course, 45 degree guns are much harder to bounce, as most lower ones need to switch to AP even at max range when fighting AWs, and I happily rush into low-angle ships when I'm an AW for just that reason (well, and the fact that most noobs don't switch to AP).

Spread, speed, or range would be valid problems. 45 degrees is not - it only dictates what type of play style the ship is best suited for, which is the same for everything from the Iowa through the Montana. 

As for how I play it, I play it like an Iowa in GB, an Alsace in W@W, or a Monty. Which is to say very defensively. I don't chase or rush unless I have a lone target I know I can take down that way, my opponents are blind, or all opponents are distracted. More often than not, however, my opponents come to me, and suffer the consequences.

As far as the EBB5.5s, only the Shinano is truly a beast. The Temeraire has the option of being slow or knocking off a turret and putting it near the bottom in firepower, the J39 gets a slight reload nerf, the Leonardo de Vinci doesn't count because it only has BB4 guns (and only 20% more than Giulio Cesare '40), and the Jean Bart has all the problems outlined here - poor range, bad spread, and relatively low firepower, but with the addition of having a design that needs the speed and agility it lacks to be that effective.
The Missouri's not much better, or worse, than any of those. They wanted to make it abundantly clear that these are inferior to true BB5s, and succeeded.

I wasn't there when these EBB were patched.
Does it use the same FCS as Iowa? What are the shells stats?

Is the spread as ugly as Iowa?