Looking to see who has one and how you like it ? I have an Octane 45 and need a 45 to use it on. I have the M&P threaded 9. Are they about the same size ? My only concern is it being too big of a gun. I wish Smith would scale the guns down a little bit.

I can get one for $600, which I thinks a good deal for an extra barrel.

Assuming you're talking about the full sized M&P 45 it's really not that big of a gun.
Not my photo, but here's a full sized 40 (same as 9) next to a 45


I've had the mid-sized M&P 45 for quite a while and shot it suppressed some (don't buy one, no one makes a threaded barrel for it anymore)
It does pretty well suppressed though I have to say it's kinda like they made 8rd mags that fit 10rds. Getting a 10th round in the mag is stuff when they're new and gets better, but I deal with failures to lock open on this gun more than any other. Especially when shooting suppressed and the mags start to get real dirty. The factory 14rd mags however work great, I just hate the plastic boot around them and it was expensive getting 10-8 basepads for all of them.
If you're looking at a factory threaded barrel package it's a pretty good way to get a threaded 45.  
In retrospect I wish i'd gotten the full sized 45.

If you REALLY like the M&P platform then dont hesitate. Mine runs great paired with a AAC TiRant 45.

How come Smith doesn't put high sights on these ?

The Springfield looks like a better buy. Cheaper, yet you get tall sights , 3 mags and an awesome case with a mag loader and holster.

No high sights on either my 9 or 45 tb kits.

There are a lot of options out there, all depends what platform you like to shoot with. My issued weapon is a MP .40 so going to different calibers on same platform is second nature.

The M&P45 has a slightly longer slide and barrel and the grip is/feels a bit deeper (longer front to back) compared with the 9/40 models. Not a big difference. I have a M&P45 Threaded Kit and it works well with my TiRant 45. I do wish taller sights were available though.

Yeah, taller sights is a no brainer. Makes you think what some companies are thinking sometimes. Like selling a car with no tires. Hmmm