Even if Silver was really stronger than Gray back then, that's not an excuse to say "since we didn't see Silver's limit he might very well be stronger than Invel", because we could also say "He may have been stronger than Acnologia as well" following the same logic.
Invel was way stronger than Gray, so even if Silver was stronger than Gray as well, he might still not be at Invel's level. Invel could probably take on any Tartaros demon alone, Mard included.
And this is talking as if Gray was as strong when he fought Silver than he was when he fought Invel, which is not the case either and his growth further supports that Invel stomps.
Invel was way stronger than Gray, so even if Silver was stronger than Gray as well, he might still not be at Invel's level. Invel could probably take on any Tartaros demon alone, Mard included.
And this is talking as if Gray was as strong when he fought Silver than he was when he fought Invel, which is not the case either and his growth further supports that Invel stomps.