I always laugh my ass off when a 1400 owner makes that remark. Hell, modify ANY bike and you can make it faster, so claiming that a modified 1400 is faster than an unmodified 800 is not much to brag about.
Is it true? Yes and no. If the uncorking is only partially done (aftermarket pipes, filters, and a re-jet but the air boxes not completely removed) then the 800 will still be faster. Or if the jetting is not right, uncorking can be a disaster: I have seen plenty of people attampt too uncork their 1400 only to end up with a bike that is slower than stock, or won't run in the rain, or stalls in a strong cross wind; power tuning is an art, and most folks don't know what they are doing.
But let's say you DO know what you are doing. If you spend the extra $2000 for the larger bike, spend another $700 on aftermarket pipes, K&N filters, a Stage III jet kit, then spend the afternoon cutting up and removing your airboxes, and manage to get the jetting just right, you can have a bike that is a little bit faster than a dead stock 800 costing $2700 less. A properly uncorked 1400 will outrun a stock 800 in the standing quarter mile, and it will gain some top speed also, so it will outrun the 800 above 100 MPH. But still does not have the lean angle to stay with the 800 in the corners, and the 800 still accelerates more quickly out of the corners, so the stock 800 can still have the advantage if the 800 owner chooses a twisty course and avoids the highway.
And, of cource, the 800 owners still has that $2700 left over to modify thier 800 with, don't they?