と言ってもいい【といってもいい】

と言ってもいい means "you could say", "it's fair to call it": you put forward a description that is a little bold and stand by it. Literally "it's all right to say". It follows a plain-form clause; a noun or な-adjective usually takes だ before と.

The claim is usually a strong one – the best, a genius, nearly done – and と言ってもいい lets you make it without sounding as if you are stating plain fact. だろう, でしょう or とおもう思う after it soften it further.

と言ってもいい and と言える

といえる言える presents the judgment as following from the facts. と言ってもいい leans more on the speaker: "I'd go as far as to say". For a stronger version still there is といって言ってもかごん過言ではない, "it's no exaggeration to say".

With くらい after it, the phrase measures a degree: "so … that you could say".