Well, subperventor would be OK from a strictly morphological point of view. Supervenitor, not quite, because the b of sub wouldn't usually just disappear, and agent nouns are normally formed from the supine, which for pervenio is perventum rather than pervenitum.
Yes...as usual, Pacifica, you are quite right. I do think, though, that the "b" of
sub- is assimilated before "p", which would make it
supperventor, no?
Now, it's extremely doubtful whether even the morpholically correct coinage would have made sense to a Roman, let alone the intended sense.
Sure, it is unclear whether the concept of "the underachiever" would have been recognized by a Roman of any social class. Certainly, certain types of behavior were expected from those born within the varying social classes of the Roman republic, as is demonstrated clearly enough by Tacitus in his Annals: "
...Mamercus Scaurus, insignis nobilitate et orandis causis, vita probrosus." (Annals Book VI, 29), but whether someone who had all the advantages of a noble birth, yet was not "...distinguished by his talent as an advocate" or in any other way would be viewed as an "underachiever" within Roman society, is quite uncertain. Even so, our Nicolaus is not writing for the Romans, who now speak Italian, but for himself and for others of us today, wherein we do have the idea of the "underachiever". Certainly, Latin is a "dead" language, in that it is no longer a social language, but if Latin is to be useful to us beyond helping us to think better, and is to be used by us today as we find it useful, then it seems to me that we need not, of necessity, make reference to ancient Roman concepts when trying to express our own.
To say the least, it seems a rather unnatural way to go about expressing the idea.
On that point, however, I will defer to yourself ten times out of ten...