"Unterdurchschnittlich" kann in bestimmten Zusammenhängen passen. Allgemeiner verwendbar wären IMHO "unzureichend" oder "unzulänglich".

Danke!

http://www.rollingstone.com/politics/story/16...

"The Great Iraq Swindle
How Bush Allowed an Army of For-Profit Contractors to Invade the U.S. Treasury
...
But two years and $72 million later, you deliver not a functioning police academy but one of the great engineering clusterfucks of all time, a practically useless pile of rubble so badly constructed that its walls and ceilings are literally caked in shit and piss, a result of subpar plumbing in the upper floors.

You've done such a terrible job, in fact, that when auditors from the Special Inspector General for Iraq Reconstruction visit the college in the summer of 2006, their report sounds like something out of one of the Saw movies: "We witnessed a light fixture so full of diluted urine and feces that it would not operate," they write, adding that "the urine was so pervasive that it had permanently stained the ceiling tiles" and that "during our visit, a substance dripped from the ceiling onto an assessment team member's shirt."

http://en.wikipedia.org/wiki/Golf#Par

"par" stammt wohl aus dem Golfsport.
Es steht dort für eine ordentliche Leistung eines Könners.

below par ist noch besser! = weniger Schläge zum Ziel

subpar bezieht sich offensichtlich auf "par" als abstrakte Leistung, nicht wie im Golfsport auf die Anzahl der Schläge.
"You've done ... a terrible job" [s. Beispiel]
Snowman (stehende 8) = Versager (4 Schläge mehr) wäre die nette Umschreibung im Golf.

Most analysts are predicting that growth will remain subpar, at an annual pace of 2 percent or less.

Im Artikel: Gridlock Has Cost U.S. Billions, and the Meter Is Still Running, NY Times 2013 10 16

Subpar bezeichnet hier m.E. das Nichterreichen einer nicht quantizierten Schwelle.