can this equipment handle bigger-ish beers?

hello!
before i start off i would like to say that i've been lurking these forums since about the beginning of the year and would like to say that this is a great community. so far i've been able to utilize the search and just dive through threads to find the info i needed, or at least enough to point me in the right direction and for that i already owe the community a thanks!

now on to my current concern.... i've done a couple extract brews and through the last few months have been acquiring stuff to further my brewing experience. for a steal of a deal i picked up 2 rubbermaid 5 gallon coolers. one of which i now know is a HLT and the other a mash tun. they have the false bottom and nice ball valves and everything on them, so they are good to go, now i'm trying to see if i am. from years of drinking beer, i've figured out that i like the higher gravity stuff, and so that what i'm hoping to brew. i've done a bit of research and from what http://www.rackers.org/calcs.shtml has told me, if i use a 1:1 water/grain i can use these for up like 14-ish pounds of grain.

sorry for the wall of text and on to the direct questions...
1) what, if any, are the downfalls of using a 1:1 ratio?
2) will i get a descent boil size with this method? (shooting for 5 gallon batches BTW)

also, sorry for the lack of regular caps, my shift key is finicky. hoping for the ability to produce 1075-ish beers. thanks in advance for any advice/pointers or anything productive.
cheers
ronjon

Also for what it's worth im going to be fly sparging

I'd run the numbers through something like BeerSmith and see what it says. Make sure you know the volume of deadspace from the false bottom. If you input the numbers and type of sparging it'll give you a good baseline for the amount of grain and water you need. This is to include the water to grain ratio as well. It varies quite a bit from batch to fly sparging.