Out of boredom I got an idea. Here's how BA's top 5 hitter prospects have performed since 1990 in their first season where they get more than 300 PAs
2014 - #1 Byron Buxton(no majors yet), #2 Xander Bogaerts(.240/.297/.362 0.4 fWAR, #3 Oscar Taveras(see below), #5 Javier Baez(not 300 PAs yet in a season)
2013 - #1 Jurickson Profar(.234/.308/.336 -0.4 fWAR), #3 Oscar Taveras(never made 300 PAs), #4 Wil Myers(.293/.354/.478 2.4 fWAR)
2012 - #1 Bryce Harpe(see below), #3 Mike Trout(see below)
2011 - #1 Bryce Harper(.270/.340/.477 4.4 fWAR), #2 Mike Trout(.326/.399/.564 10.1 fWAR), #3 Jesus Montero(see below), #4 Domonic Brown(.272/.324/.494 1.7 fWAR)
2010 - #1 Jason Heyward(see below), #3 Giancarlo Stanton(.259/.326/.507 2.3 fWAR), #4 Jesus Montero(.260/.298/.386 -0.4 fWAR)
2009 - #1 Matt Wieters(.288/.340/.412 1.3 fWAR), #3 Colby Rasmus(see below), #5 Jason Heyward(.277/.393/.456 4.6 fWAR)
2008 - #1 Jay Bruce(.254/.314/.453 0.7 fWAR), #2 Evan Longoria(.272/.343/.531 5.5 fWAR), #5 Colby Rasmus(.251/.307/.407 2.6 fWAR)
2007 - #2 Alex Gordon(.247/.314/.411 2 fWAR), #3 Delmon Young(see below)
2006 - #1 Delmon Young(see below), #2 Justin Upton(.250/.353/.463 0.6 fWAR), #3 Brandon Wood(never made 300 PAs), #4 Jeremy Hermida(.251/.332/.368 -0.8 fWAR), #5 Stephen Drew(.238/.313/.370 -0.7 fWAR)
2005 - #1 Joe Mauer(see below), #3 Delmon Young(see below), #4 Ian Stewart(.259/.349/.455 0.9 fWAR), #5 Joel Guzman(never made 300 PAs)
2004 - #1 Joe Mauer(see below), #2 B.J. Upton(.300/.386/.508 4.5 fWAR), #3 Delmon Young(.288/.316/.408 0.0 fWAR), #5 Rickie Weeks(.239/.333/.394 0.3 fWAR)
2003 - #1 Mark Teixeira(.259/.331/.480 1.9 fWAR), #2 Rocco Baldelli(.289/.326/.416 1.7 fWAR), #3 Jose Reyes(.273/.300/.386 1.8 fWAR), #4 Joe Mauer(.294/.372/.411 3.4 fWAR)
2002 - #3 Hank Blalock(.300/.350/.522 5 fWAR), #4 Sean Burroughs(.286/.352/.402 2.6 fWAR), #5 Carlos Pena(.242/.316/.448 0.2 fWAR)
2001 - #1 Josh Hamilton(.292/.368/.554 2.4 fWAR), #2 Corey Patterson(see below)
2000 - #2 Pat Burrell(.260/.359/.463 0.5 fWAR), #3 Corey Patterson(.253/.284/.392 0.2 fWAR), #4 Vernon Wells(.275/.305/.457 1.4 fWAR), #5 Nick Johnson(.243/.347/.402 0.8 fWAR)
1999 - #1 J.D. Drew(.242/.340/.424 2.5 fWAR), #3 Eric Chavez(.247/.333/.427 1.2 fWAR)
1998 - #1 Ben Grieve(.288/.386/.458 1.5 fWAR), #2 Paul Konerko(.294/.352/.511 1.9 fWAR), #3 Adrian Beltre(.275/.352/.428 3.4 fWAR), #5 Aramis Ramirez(.300/.350/.536 4.7 fWAR)
1997 - #1 Andruw Jones(see below), #2 Vladimir Guerrero(.302/.350/.483 1.5 fWAR), #5 Travis Lee(.269/.346/.429 1.0 fWAR)
1996 - #1 Andruw Jones(.231/.329/.416 3.7 fWAR), #3 Ruben Rivera(see below), #4 Darin Erstad(.299/.360/.466 2.8 fWAR)
1995 - #1 Alex Rodriguez(.358/.414/.631 9.6 fWAR), #2 Ruben Rivera(.195/.295/.406 1.4 fWAR), #3 Chipper Jones(see below), #4 Derek Jeter(.314/.370/.430 2.2 fWAR), #5 Brian Hunter(.302/.346/.396 1.5 fWAR)
1994 - #1 Cliff Floyd(see below), #2 Chipper Jones(see below), #3 Jeffrey Hammonds(.264/.323/.486 1.8 fWAR), #4 Alex Gonzalez(.243/.322/.398 0.3 fWAR), #5 Carlos Delgado(see below)
1993 - #1 Chipper Jones(see below), #3 Cliff Floyd(.281/.332/.398 0.4 fWAR), #4 Carlos Delgado(.270/.353/.490 1.6 fWAR), #5 Tim Salmon(.283/.382/.536 4.7 fWAR)
1992 - #4 Chipper Jones(.265/.353/.450 2.9 fWAR)
1991 - #2 Andujar Cedeno(.283/.346/.412 2.9 fWAR), #3 Ryan Klesko(.310/.396/.608 2.1 fWAR), #4 Jose Offerman(.260/.331/.333 0.6 fWAR)
1990 - #3 John Olerud(.265/.364/.430 1.4 fWAR), #4 Juan Gonzalez(.264/.321/.479 1.9 fWAR), #5 Sandy Alomar(.290/.326/.418 2.4 fWAR)
This is a bit skewed as Andujar Cedeno, Jeffery Hammonds, Ruben Rivera, Aramis Ramirez, Jose Reyes, B.J. Upton and Dom Brown had a few call ups though most of these guys were young upon being called up rather than college age players. By in large, the players typically hit for decent average/on base but often their power would come down some. There were a couple super star types right away. There were a few out right busts. Overall though it appears that most were average to above average.
For reference, I chose 300 PAs because that's typically more than your late season call ups. Bryant is probably more of a 400-500 PA type for 2015.
Edit: left out jeter by accident