I have a daiwa underspin US-40XD and I was thinking about putting braid on it.  It holds 85 yds of 4lb mono and thought I could get more line on it, and line strength by going a thin braid. 

I didn't know how braid would work on a closed reel.  Any thoughts or suggestions would be welcome.  Thanks

It will work just fine. I used braid on my wifes closed face reel and had no problems at all.

I have several closed face reels with superline for inexperienced angles that may fish with me.

You will get the best casting performance with the stiffer types of superline like Original Fireline, Stealth, etc.

I bought a used closed face Johnson Country Mile for kids to use that had Fireline on it. It worked decent but no better than mono. I've put small powerpro on a buddy's Zebco 33 and it hung up really bad. However, that reel is designed around ten lb. test.