To answer this question I'll take the top 10 position prospects from the Pirates Prospects Position Guide and look at their on base percentage, slugging percentage, and OPS. I'll also try to offer some context by looking back to last year.
Starling Marte, Altoona, AA, (OBP = 351/SLG =455/OPS =806). For the last ten games (283/341/624). This performance nearly equals last year's, although July has been a bit ugly. Still a nice line, and the slugging is up a bit (23 doubles and 5 HR versus 19 and 3 last year). Here's a quote from BA,
Marte's defense will accelerate his rise, as will his intelligence and ambition. But his strike-zone discipline will have to keep pace. Ticketed for Double-A in 2011, he could push Andrew McCutchen to an outfield corner when he arrives in Pittsburgh.
His strike zone issues do not appear to have been solved, though AA pitchers are not able to take advantage of them.
Tony Sanchez, Altoona, AA, (346/314/661). For the last ten games only (318/333/652). This is disappointing, as the OPS is 200 points below last year's. The hitting will have to improve for him to remain a top prospect, although the glove should carry him to the majors in some capacity.
Andrew Lambo, Altoona, AA, (265/307/572). For the last ten games (348/439/787). Well, there are a few hopeful signs recently, although this has been a bad year for Lambo. This is his third year in AA, and it's hard to consider Lambo a top prospect anymore (especially since his tool is hitting).
Chase d'Arnaud, Pittsburgh Pirates. Not much to say. He may not be up to stay, but last year's poor performance is no longer a concern. (Note, he wasn't hitting a Indy when he was called up, so his struggles with the Pirates were to be expected. His season stats at Indy were fine, though: 347/418/766.)
Mel Rojas, West Virginia, A, (298/345/643). For the last ten games the numbers are better, but still not outstanding (318/634/952). Two doubles, two triples, and three home runs in the last 10 games suggest that the power tool is starting to emerge. He had no home runs or triples last year.
Jarek Cunningham, Bradenton, A+, (323/534/857). Fifteen home runs this year, and he's been on fire during the last 10 games (400/545/945). He has definitely moved up in the rankings. Looks like a pretty good 18th round pick up.
Exicardo Cayonez, ?, (189/063/252). As Charlie noted a couple of days ago
after going 2-for-32 for State College, he's back in the GCL.
He actually is not on that roster yet, but this has been a brutal year for him.
Gorkys Hernandez, Indianapolis, AAA, (350/409/759). In the last 10 games he's been almost as hot as Jerek Cunningham. He hit a home run!!! and had a slash line of 390/579/969, which is excellent. Given his excellent defensive skills, this makes Hernandez a legitimate prospect again. He's in AAA, dong well, and is 23.
Brock Holt, Altoona, AA, (338/382/720). He seems to have heated back up in July going 341/405/747, so he's in the picture, but not dazzling. This is not as good as last year, (410/438/848), but is still decent.
Only Cayonez and Lambo are raising serious doubts, and one hopes Cayonez can bounce back.