The 'brain wiring' is something that I'm arguing to be over 95% environmental, whereas genetics play a minor part.
In a sense, all Humans (barring any genetic issues that actively inhibit learning, memory retention or could cause brain damage) would be born with an equal potential for adaptation and learning, therefore, what they actually become in life is basically a byproduct of the environment (exposure to information/stimulus).
Intelligence wouldn't be a genetic factor, but rather environmental one that hinges heavily/entirely on ones exposure to relevant education, experiences and knowledge - there is no 'pre-determined' knowledge or skill-set that one is born with (let alone 'instincts' - which are very different from motor functions and reactions to external/internal stimulus).
There might be (probably are) genetic tendencies, but having a tendency towards something is NOT a guarantee it will develop in an individual - that much depends on the environment.
For example, a genetic predisposition towards violence will not inherently result in a violent individual if they are raised in an environment that isn't abusive (actually, they will be LESS prone to violent behavior compared to others in such an environment).
Equally so, just because a person is abused as a child doesn't have to necessarily grow up into a sociopath or be a danger to society if they were exposed to (for example) the notion they could have a better life and were prompted/encouraged to take choices in life that might eventually lead them to that life.
But if a person is basically exposed to just 1 thing and nothing else, chances of them knowing any other way of life or envisioning it in the first place is next to impossible because they would have 0 frame of reference for it.
The blank slate aspect works well in this regard because, how exactly do you expect of a person to comprehend anything unless they were exposed to concepts that would allow them to create a frame of reference that would begin to even come close?
For that matter, how would you expect of a person to score high on an IQ test if they were never educated on how to add, subtract, multiply, divide, know what basic geometric shapes are, etc., or which colors have appropriate names... or what more complex geometric forms are, more complex words, equations etc...?
If a Human is not exposed to these things and actively encouraged to use them (in a way that is interesting to them), you cannot expect them to score remotely close to 'average' (let alone 'high') - and society could easily classify this person as 'slow' or 'stupid' simply because they were not exposed to the information that would allow them to understand, let alone process (or encouraged to use) this information.
That has not been established.
I would agree that it hadn't been established... directly.
There was an episode in Voyager ('The Raven' I think) in which 7 of 9 (when in a shuttle and on her way to the Borg signal) addressed Tuvok in the following manner:
'Vulcan. Species Three Two Five Nine. Your enlarged neocortex produces superior analytical abilities.'
But this wouldn't necessarily translate to Vulcans being 'smarter' (they DID nearly destroy themselves and had a pretty violent history) - and it could just as easily manifest only in an environment that actively encourages usage of analytical abilities- which might not have been the case until VERY late in Vulcan cultural development... it also depends on which species was 7 actually comparing Vulcans to, or what kind of 'standards' the Borg might have in the first place (but the Borg WOULD designate such biological traits as 'superior' - Humans or Vulcans might not - at least not the Vulcans from the 23rd/24th century, the 22nd century ones though might be a different story...).
Aside from that (which I don't think would necessarily carry any weight), I don't think we have any concrete evidence to support the claim that Vulcans are biologically smarter than Humans. As stated though, their education 'might' be, but we've also seen very little of that to make any final determinations, and lets be frank... we've seen very little if anything of the Human educational system - which wouldn't be anywhere NEAR in shape or form of what is currently being done given the cultural changes and overall approach to life humanity underwent according to Roddenberry).
Vulcans have been established to be 3x stronger though, but that's not the same thing (and it doesn't make them necessarily 'superior' - merely provides an advantage under certain conditions, just as Humans might display attributes under certain conditions that could seemingly 'surpass' those of a Vulcan - and we don't know how much of that strength differential is due to actual biology/genetics compared to gravity and overall eco-system of Vulcan as a planet and whether humans could develop similar attributed training under same conditions for certain periods of time).