It sounds to me like you are recording PA systems using inexpensive clip on mics. Is that right? [EDIT TO ADD: Yes, you are]
A Nagra recorder is no doubt superior to a Sony M10, Edirol, Tascam, etc. But given your mics and situation, will that superiority matter? I'd say probably not.
Much like the difference between a Sound Devices or Nagra large size recorder and running an outboard preamp and smaller recorder, absolutely, the SD and Nagra boxes are superior and offer compelling features like (a) better reliability, (b) better durability, (c) quieter stock preamps (depending what outboard preamp you are running, of course). But unless you just really like the sound of their onboard pres, even if you're recording with DPAs or Schoeps, I'm not sure they're really worth it, strictly speaking, for recording PA systems with the gear you have.
In your particular case, where we're talking about stealth recording with the gear you have, I think that's even more the case. The Nagra is much better than a Sony M10 (for example) in theory but the M10 is also reliable, sounds pretty good, and is hundreds of Euros cheaper (and I think it's smaller).
It'd help to know what mics you're running, but if you are not currently running superpremium mics, I'd invest in that long before I'd blow money on a big name for a standard 2-channel non-P48 recorder.