For all the flack Kishimoto gets about his writing of female characters, he cooked with Kushina. She's below OM (Orange Mask) Obito for sure, since he managed to beat her when he was far younger (crazy to think he was only a teen when he attacked the village 😭) but in terms of beating Pain, I think it's plausible that she could do it. She managed to wrap Kurama in her sealing chains faster than he could react two times, and both times she didn't have access to her full power. In Pain's defense, when he had to fight 6-tailed Naruto, he was fairly weakened by then, but I don't think he would end up winning against a fully transformed Kurama even with his full power. As IKTime showed with his scans, Naruto himself said that Kushina was stronger than him even with his newly improved Sage Mode, and in Naruto's fight with half of Kurama Naruto was losing until Kushina used her chains to hold the beast down and that allowed Naruto to barrage him with attacks so he could pull his chakra out. I'd say that Kushina was the main reason Naruto won that fight with Kurama. I would agree with putting her over Pain and WA (War Arc) SM Naruto.