no i agree that tails correlate with increase in power but only for kn0-8 forms. narutos new bijuu mode is not a kn form.
The number of tails correlates with power ,the bigger the number of tails the closer he is to the full power of Kyubi.Knowing that this recent form is the Kurarama giving Naruto his chakra there is no logical reason why his power would be lower.
and so what if the chakra can turn solid? most chakra is solid to perform attacks but my point still stands that it's just a chakra coating unlike the kn forms and anyother bijuu on the field. there's also the fact that you see two naruto in it and he put gai and kekashi in it.
This is important ,knowing you acknowledge the fact that tail number correlates with power then you must logically acknowledge that the current form is superior as it as shown itself to possess 9 tails and it was expressively stated to be the result of Kurarama giving his full power to Naruto .The only argument you could then make would be the fact that the form isn't solid to prove it is unable to withstand the level of attack the 8 tails did, as you accept the fact that the full power of the 9 tails surpasses the 8 tail then it should logically be more resistant.
also the kn8 form is superior due to durability being mountain level and mountain destructive capacity (seeing as with only six tails he did a mountain level attack), but this could change soon.
You do know Naruto produced a blast in the Kyubi chakra form that defeated the blast of 6 bijuus and that each of them were capable of blowing up mountains?