Just this past Shabbos I was learning some Sifsei Chaim and he brings the gemara in Berachos that says that
“all [the good things- Rashi] that the nevi’im prophesied was only for ba’alei teshuvah, but for tzaddikim gemurim ‘ayin lo ra’asa elokim zulasecha'”
He explains that a ba’al teshuvah gets extra siyata d’shamaya to grow because Hashem wants the person to come back and without the help the struggle would be too hard.
But a tzaddik gamor doesn’t get this siyata d’shamaya- his growth is with his own strength so to speak and therefor his reward is that much greater.
So even though “b’makom she’baalei teshuvah omdin tzadikim gemurin einom omdin” and therefor a tzaddik gamor may not reach a higher level than a ba’al teshuvah per se. But since his growth was of his own accord ‘l’fum tzaarah agra”.