Advice after mods

So, what I have done is this... WR250R 11,000 miles on it
-Opened up air box
- K&N hi flow filter
- FMF pProgrammer
- Megabomb header
-Powercore pipe
- .8 lb lithium battery
- pair of Mitas RockEater tires

What Im more concerned about is the whole exhaust and air box mods.
I am asking more of the engine now and I am wondering whether I NEED
to check the VALVES. Consideriing that there is much more air, fuel, and HP
being put through the machine. Is a valve check necessary. What else should
be checked after these mods? thanks for your advice!

Ride it.

I prefer oiled foam air filters for off-road bikes.
I also believe in servicing the air filter on a bike ridden off-road very often, and using a real foam filter oil.

As far as checking the valve clearances, assuming the bike hasn't been thrashed to death and/or inhaled a bunch of dirt through the engine via through the air intake so far, it's more than likely OK.
But, if I'd just picked up a WR-250R with 11,00 miles on it, I would check them to know where I stand and for peace of mind.

Speaking about checking valve clearances:
I just checked mine on my 2009 Yamaha WR-250F today (different bike, by the way).
Still in spec. after two seasons of off-road use.
In fact, I don't think they even moved at all.