'Whom' is an object pronoun; 'who' is the subject. Let's be wary of whom we accuse of 'pretension', having taken such obvious pains to assert our own linguistic legerdemain. Otherwise:
I absolutely concur. I might qualify for MENSA; it would do little, however, toward demonstrating academic potential. Indeed, it is trite that MENSA's 'active' constituency comprises either such monumental prodigies as will have enjoyed effortless distinction in all other walks of life and, compelled by a sense of 'noblesse oblige' [their own narcissism], wish to espouse a society dedicated exclusively to the celebration and promulgation of human cultural advancement [themselves] through intellectual refinement [posturing]; or disaffected intellects of marginal standing and meagre accomplishment, who seek a kind of approbation and peer-validation that they cannot reasonably expect elsewhere; or an amalgam of the two: gifted but ineffectual misanthropes that are too consumed by ennui, cynicism or acute neuroses to pursue the careers in law, medicine and/or quantum-physics that they were otherwise born to. The archetypal (okay, stereotypical) 'mensan' left school with a constellation of GCSEs and a triumvirate of A-Levels, only to flounder at university because they couldn't negotiate living independently/were too-easily distracted/prone to tangential digressions/ruminating on Proust, and found themselves shortly thereafter eking out an ignominious occupation conducting buses or teaching at inner-city schools. They joined MENSA in order to fraternise constructively with people of (broadly speaking) university-level intelligence, absent the raft of obligations concomitant with university attendance. (At least, that's why I'd join.)
Your modicum of academic potential will already be apparent from your A-Level predictions. It is axiomatic that whilst virtually anyone of reasonable intelligence will succeed at A-Level, not everyone who succeeds at A-Level is (broadly-speaking) 'intelligent', at least not in a manner conducive to 'redbrick' education: hence interviews; because the premonition of someone whose own 'intellect' has been already sanctioned and affirmed beyond reproach is probably as reliable a barometer as any. Thus, to attempt artificially tipping the balance by citation of vague and spurious, if not altogether unaccountable, evidence for some nebulous notion of 'intelligence' possessed of yourself would betray sheer arrogance. Personal statements are supposed to exhibit character; they are complementary to what is an otherwise one-dimensional, academic profile. By all means allude to the time when you orchestrated a three-hundred man 'chess-off' at your local MENSA chapter, to which qualification would be an incidental pre-requisite (because some people's 'character' is, after all, intellectual; and academic intellectuals are the bread-and-butter of Oxbridge), but don't expect that they will apportion to it any more weight than the prospective mathmo's professed affinity for one-digit cryptic su-doku.
Indeed, as someone mentioned previously, citing MENSA membership as though a bona fide certification of intelligence (even assuming 'intelligence' were the sole criterion for admission to Oxford/Cambridge) would probably suggest naïveté more than anything else. Why should the admissions tutor dispose himself any more favourably toward your application as a result than would a MENSA adjutant toward someone, who possibly fell short on the I.Q. front, for having scrawled 'I got into Cambridge!' in hysterical, italicised font across the front of their exam-script? The two simply don't equate.
Of course, were MENSA an exam-board, and a score of '130+' deemed equivalent to 'AAA', perhaps it would be different.