Why did you need to increase the transmit power anyway? Unless there is a problem with it, you generally don't need to mess with it.

Increasing the range is pointless if at the stock power it can cover the entire house anyway. All you're doing is generating extra heat, using more power and sending the signal further out into the street.

Also, just because it lets you type in the maximum of 251mW doesn't actually mean it works properly at that level or even actually transmits at that level. All the Tomato web interface is doing is passing your settings to the Broadcom utility that sets the parameters which isn't particularly intelligent about what it lets you set. The utility lets you do things that you're not supposed to do and assumes you're following the limits for the particular hardware you've got. The radio is designed to operate at a specific transmit power and turning it up may have adverse affects on the signal to noise ratio or spectral purity.