I doubt the XF-9 will be used in the end in any plane. To me it proves that Japan is fully capable of going on their own to develop jet engines in the F-119 class, but because they are "forced" to buy the F-35s, because of their relationship with the US, it won't ever even be used in any plane.But there was a news that Japan was seeking cooperation with Rolce Royce of UK for the XF-10 engine. That would mean that a testbed XF-9 isn't a mass production engine. There were also some mentions of SiC SiC Composites to further achieve better heat tolerance and therefore efficiency and thrust rating.
IMHO, the Japanese XF-9 is a testbed to develop certain advanced technologies (high electrical power generation, composites etc) but I doubt if Japan would go solo for equipping its F-3 fighters with IHI engines. The better results XF program gives Japan, the better its position in negotiations with a foreign partner.
I'm sure Pratt and Whitney won't pick up IHI for a JV. They'd want to sell Japan an entire engine. Idk about GE, but I don't think GE is that interested.
But a Japan - UK JV is on the horizon and it does sound like it is going to materialize. Especially, as of recently UK and Japan went through the sale of ARM to Softbank.
My bet is on a IHI - Rolce Royce JV. This means that Tempest and F-3 would sport the same engines (or share a good deal of technology). Rolce Royce is also well established in mass producing and servicing engines so Japan would want such a partner rather than solely bet on IHI.
Same with the UK, rolls-royce basically has no business from the UK airforce, because they've decided to buy the F-35, since it cost too much for the UK to develop their own fifth gen fighter. I doubt rolls-royce will partner with IHI on furthering the XF-9 or XF-10, there just isn't demand for such an engine, especially since both countries have chosen to go down the F-35 route.