Well, if he didn't get stronger than he will be one-shotted by Elite (not Yhwach). Yhwach would need even less power for him, if that's even possible.
He did get power up, that's for sure, since he was with Urahara and co. Chad and Orihime also got a lot stronger.
But I don't want to see it. Since it will really be another one unlogical power up. Renji not even captain level prior to RR got to Kyoraku's and Ukitae's level of power. Grimmjow was multiple times more powerful than Renji, so if he get power up, what to expect? Aizen level? I really don't like that idea.
Out of all Quincies, only Yhwach (FV), Hashwalth, Uryuu, Lille, Pernida, Askin, Gerard are left. And they are reserved for Ichigo, Tenjiro, Kirio, Shutara, Ichibei, Ichigo, Yoruichi, Chad and Orihime... That's not even couting Urahara, Tessai, Isshin, Ryuuken, Kyoraku, Ukitake, Shinji and others... If Grimmjow gets back, as you said he only can lose a fight. Will fans be pleased with that? I don't think so.