I ran the masteries and rune pages in this Ryze guide: http://www.mobafire.com/league-of-le...-scroll-155276

Here are the numbers with Sorcerer's Shoes:

Out of Combat: 416
In Combat: 409

Sorcerer's Shoes with Alacrity:
Out of Combat: 429
In Combat: 423

This doesn't make sense. 409+15 = 424. I can blame that on some in-game rounding but 416 + 15 = 431 and no amount of rounding makes this one go away.

These issues may be due to bugs in the way the masteries and runes work together. For instance, it's also strange to me that with Alacrity, the boost from 3/3 in Wanderer is only 6 ms but it's 7 when the boots are unenchanted. Wanderer is suposed to scale with your ms since it provides 2% increased ms out of combat (which they really should define in an objective way). Thus, I don't see how it could be LESS of a bonus when my movement speed is actually higher. If there is some rounding going on, I'd expect it to go up rather than down. Plus, Alacrity is supposed to be a flat +15 ms so why would the rounding be any different.

So is there a Riot guide somewhere that tells us what stats have diminishing returns or limits?

Seems kinda bad if not...

I wonder what would happen to the game if diminishing returns on movespeed were removed?

That guide is antiquated as it still talks about MS 1-5 (the old system). Honestly they should remove the MS cap or at the very least rework things like the alacrity boots because they almost never will actually give the advertised move-speed. When things like this are 'hidden' in some guide on the site and not explicitly stated in the game client it is kind of lame. Perhaps add this to the descriptions in the store?

What about summoner spell cooldowns? Are there any caps there if I use Distrotion + Summoner's Insight + 3/3 Mastermind?

Flat MS bonus' should not be effected by the softcap only % increases. That sorta defeats the purpose of getting Alacrity on mobs or swifties.