I don't think it's likely, even with tuned custom profiles, that the new P600 can challenge the 9900 for colour gamut. The P600 reverts back to Epson's "classic" 8-ink mix (not counting the extra PK vs. MK black), and the only serious claims about the ink set I've seen so far include reformulated black for slightly better Dmax and improved resin encapsulation presumably for an unknown degree of reduced clogging.
The 9900's ink set is older, but offers a 10-ink mix, adding Orange & Green to the other 8 of the classic mix. If you look at the gamut plots you made where the 9900's wire frame exceeds the P600's solid core, these are colours that mostly make sense for a printer that can call on dedicated Orange and Green pigments. The 9900 shows better gamut mainly in oranges, yellows, greens, cyans and blues, most of which can benefit from those extra couple of dedicated pigments.
Also consider that the P600 is the most entry level of the new SureColor printers, and as such probably has some hardware limitations that may impact the colour gamut that can be delivered. For example, the P600's head has only 180 nozzles * 8 ink channels, whereas the 9900's head has 360 nozzles * 10 ink channels. I'm betting as a fully "pro" large format model, the 9900 also has much more accurate head placement and paper transport. Now, the P600 has a minimum of 2 picolitre ink droplets vs. the 9900's 3.5 picolitres, but there's no info I can see on which inks of the P600 can use droplets that small. Possibly only the new blacks (hence the great Dmax). So for colour, I'd say it's quite possible that the 9900 head has the ability to lay down a more accurate and more complex ink dither pattern, which may translate to better gamut in some hues where the driving factor isn't just the extra 2 pigments.
Interestingly, the SureColor P800 which has a larger size that presumably should be able to incorporate a little more advancement, also has a 180 nozzles * 8 ink head design, but claims 3.5 picolitre ink droplets... like the older models rather than 2 picolitres like the P600.
At our print shop we're rocking the older Epson 11880's (plus one old 9880 converted to K7 carbon ink), and have no plan to move off of them until they're no longer maintainable and ones in good condition are no longer available. The prints we produce consistently meet or exceed the expectations of our customers on every level. Printers are like digital cameras -- almost all of the last 2 - 3 generations (at least) of these machines are better than what 99.9% of people need.
For the vast majority of us even at the most enthusiastic levels
, as long as our gear stays working, and isn't actively blocking us from great image-making in very specific situations, then we can stay off the hamster wheel of equipment upgrades. I'm almost immediately skeptical of most new product announcements these days, unless there's something clearly net new & ground-breaking trumpeted immediately.
That's not good news for vendors who just want us to upgrade to new machines that are essentially re-treads of old machines. To get more of my money, they'll have to really innovate, or else wait for my gear to die off and require replacement. The latter is what I'm most worried about, observing the trend lines of reliability, maintainability and service over the past few years...