It should be a great choice for your purposes using an iPhone/IOS.
You might check out the IOS centric
review of the BTR3 that was just posted on ilounge.
They rate it A- and suggest some pluses in using it with an iPhone:
1 - Independent 30 step Bluetooth Volume control. The review says IOS Bluetooth volume control is limited to 16 steps. This can be a problem with sensitive IEMs as each step will boost or cut the sound quite a bit so it can be difficult to dial in what you want. With the BTR3 you should get both the finer 30 step volume control built in to it, plus the 16 step in the iPhone (use one or the other or both). This could be an important advantage for IEM users.
2- Support for the less lossey AAC codec.
Some of negatives would include, iPhone does not support any aptx flavors...so you will not get Aptx HD or LLC Benefits. I do not have an iPhone so I have not personally tried this combo or know how well the Fiio Music Player app for IOS works. Perhaps others can chime in on that.