I have started experimenting with a Enya 40XZS on a pipe.
Engine was run in on bench with 20~25  3 minute runs, and then flown about 30 times with stock muffler.  During those 30 flights, I was teaching my nephew inverted flight and he put it in the dirt, bending the original needle.  I replaced the NVA with one of Randy's PA units, and other than that the engine is stock.

PIPE Set up:
Profile Tanager, around 50oz I think.
Pipe is one I bought used, it was advertised as a "40-51 size", inlet is 5/8" ID and outlet is 3/8" ID.  Has a pressure tap midway of the main taper section.
I think it is one of Randy's but not sure.  Pipe is set at 17.5"
Using Sig 10% Champion, uniflow tank no pressure, Sig RC long plug, APC 11-4.

First pipe run, ground launch 9.5~10K.  After launch engine really leaned out to a screaming 2-cycle, I'm guessing 11.5~12K, maybe more.  Running 4.3 laps on 63' lines eye-eye.  She was hot when landed, I was a little concerned about damage, but it seems to have survived the abuse.     After 10 minute cool down, opened needle 1 full turn and re-started for next flight.  On ground it was running in a rich 4-cycle, (8000 or less) so slow I didnt even bother to tach it, just went ahead and launched, thinking it would still be plenty rich in flight and I would sneak up on needle in subsequent flights.   To my surprise flight speed seemed close to normal, around 5.1 laps, and it sounded to me around 10k.   Toward the end of this flight I blew the gasket at the header flange,  so that's all the testing so far.

What I dont understand is the large change in RPM from ground launch to flight,  which was around 2000 rpm in both cases.  This is my 1st experience with pipes, but I've been around them at contest and it seems that the more normal change from ground launch to flight is something like 500 rpm.  Is that right or am I missing something here?  The other question is , do most of the pipers use tank pressure or not ?  I was thinking of trying that next to see if it might minimize the change in RPM  ??  

Stock Venturi is .275 (7.0mm) with .155 spray bar.     So area is only .0167" to start with..

Not a sure diagnosis here but I would check for a pinhole (or smaller) air leak in the system especially as the pipe was used. A pressure test is called for. Any air leak in a tuned pipe set-up can result in erratic runs. 

May get some debate here but by my observation more folks run them vented than using pressure. Dumb question but have to ask: Is the pressure fitting capped off?  2000 RPM difference between ground and flight really is excessive so you are right in suspecting that something "just ain't right".

You are probably a member so have already seen it but in the the 2009 Special Edition of the PAMPA Stunt News you will find a must-read article on tuned pipe set-ups written by Brett Buck and Dave Fitzgerald. Lotsa good info there.  

We've had some recent failures with those cute little yellow plastic caps, in case you're using one of those. It is conceivable that a piece of fuel tube with a screw in it could leak, tho I've seen experienced fliers use that method. A BB or 1/8" rivet in a piece of tubing is a better bet, IMO.  Steve

I use this clear plastic 360 degree protractor as a degree wheel.

Actually, the article you mention was in the PAMPA newsletter (magazine really). Special 2009 issue. Lotsa furriners are members of that organization. An internet-access membership is actually pretty reasonable for the info you get. In the meantime if you send me your address directly I'll copy off the article and send it to you directly.