I'd like to get one for my Remington 700 5r in .300 Win Mag. However, reviews seem to be almost non-existent. Does anyone have any thoughts/experience?

If you want a chassis get an MPA.  They have the best feature set and modularity. I don't think I've seen more than one of the orias in the wild. 

My only person experience comes from two fronts.  I have built two Mega MA-10 rifles that, in conjunction with JP core components are incredible performers.  I am a big fan of Mega's commitment to design quality.

I have handled Orias chassis ( which is made by Mega Arms ) in Bergara rifles and I really like them.  I do not like chassis guns as a rule BUT....the Orias is light( as it could get, I guess) and felt solid as all get out and very comfortable.  I would have ZERO issues running the Orias, personally.

Second the MPA.  Fantastic balance, light, and great ergos. A bit spendy but worth it in my opinion.

I run one for my competition rifle.  My shooting partner runs a MPA.  here are a few comparisons.

Pros for the Orias:
Ergo is really nice with curved edges if you plan to grip the chassis just forward of the magazine
Ability to choose whatever buttstock you like
if you torque the screws in the recommended order, zero POI shift when removing the barreled action
weight "feels" lighter than the MPA
Ability to add QD points to the side of the forend
Built in QD points

Cons for the Orias:
the one pin barricade stop in front of the magazine is not ideal (I use a RRS mount which is much better)
does not have the same modularity on the bottom of the forend as the MPA
Built in QD points are a little close together for my liking
Wish the Keymod/M-Lok slots on the bottom went all the way out to the end instead of having a built in sling swivel

Honestly, I love both chassis's and I can't really tell the difference when I get behind either of them.  I do like how all that parts on the MPA work together.

Here is a picture of my orias:

SEXY!! Does the Orias require a seperate folding mechanism like the one from LAW tactical?

wondering if this would work with a Howa 15000 short action

Yup.  bought mine from MDT.  Once folded, it does flop around unless you secure it where the MPA locks in the folded position.

I'm a big fan of the MPA... if you order one be super specific about the features you want, there really is no standardization, if you're not careful you may or may not get the features you want.

I'm not sure what you mean by that they have a couple different models, but the features are pretty standard, then there are more accessories you can add.  The comp model has a magwell cut on it and comes with a couple accessories, but most of the stuff is pretty modular as far as that goes.