as it grows from kills, can it exceed 2.5?? I've been playing with sivir and ashe alot and focus on att spd with their builds. I get 22% from runes alone, and then a bit more from masteries, making it very easy to get very near max with items, usually around 2.42 or something like that. I'd like to build varus for att spd, but if his natural att spd growth can't exceed 2.5 than that trait is kinda wasted. However, if I leave room for it to grow, then I have to put up with slower att spds WHILE it is growing which I dont like either...

no

No, you cannot. And if you build any champ 100% attack speed, you are wasting their potential. If you are playing an AD Carry such as Varus or Vayne, that you may think scales well of attack speed, as a guy who mains AD Carries I'll let you know it is much more effective to run Damage / Armour Pen runes, and buy actual AD items.
Try going Infinity Edge -> Phantom Dancer instead of the other way around. Remember, spells scale of ad no attack speed, and they are most of your poke/harass early on.

Its better to but your big damage items early. Rather than attack speed.