has anyone realized a ceiling on HH improvement for certain attributes? i'm specifically asking about ath, spd and def. is there any possibility that a recruit will realize per/lp'esque climbs or or we talking about a more modest 28-40ish range?

I haven't found any hard caps, but in my experience, ATH has the most modest gains of any skillset. I'd have to check to be sure, but I don't think I've ever seen ATH break 40 points of improvement. Usually, even high-high ATH has improvement between 27 and 35. 

SPD I've definitely seen break 40, but I don't recall seeing it break 45. 

DEF is more like SPD. I know I've seen it hit 45, but I can't think off the top of my head of a guy who broke 45 points of DEF improvement. 

There was just a great thread on this.....I'll see if I can find it.

Most defense I've had a guy gain is 46.  Same guy also gained 40 in both ath and sp.  He was a freak of a guard for D3, but this is the only time I've had numbers like this.

Here's the thread scaturo was referencing.

I have a guy whose cap gets him a 48 point gain in defense. 1 & 4/5 seasons to gain the 9 points. I feel like I've seen 50 but can't come up with any examples.

edit: just saw the chart. Interesting stuff.