I do fine-art photography and photo-art (photos & digital art combined). I am now producing gallery prints up to 17"X25" on an Epson 3880.
An Epson 3880 inkjet is one of the home/studio printers that can indeed produce colors outside of AdobeRGB (aRGB). So you have two choices: stick with aRGB, ensuring that what you see is faithfully reproduced on paper, but possibly giving up a few tonalities: or move to a larger color space, ensuring that those few extra wide tonalities make it to paper, but possibly resulting in tones on paper that you would not have expected from looking at the monitor. The former will save you ink and paper. The latter is potentially incrementally more 'colorful'.
By using a calibrated wide-gamut monitor and editing RAW files in AdobeRGB, I was able to print very close to what I was seeing on my monitor (prior to an output adjustment for substrate and 3500K gallery lighting). That saved a lot of time, ink and paper. Much of my work contains color gradients, shades of blue skies, and deep red sunsets running the entire length of the work. Thus my need to edit 16 bit in the ProPhotoRGB color space to produce accurate color and avoid banding or blocked-up color.
Imho ProPhoto may indeed produce incrementally more colors, but it will do nothing to help you avoid banding - just the opposite (although you should have no such problems working with 16 bit files coming straight from Raw data). The key is ensuring that your software, printer drivers and printer all work at the highest bit depth possible, ideally all at 16 bits (video drivers, video card, LUTs, monitor panel as well).
But now with the need to edit in ProPhotoRGB...
There is no such set need, it is just a personal choice with the compromises outlined above. I would try ProPhoto/Melissa/BetaRGB and aRGB on a few of your reference prints. If you cannot see a relevant improvement, I would stick with your tried and true workflow.