A number of years ago I remember a 900 mhz cordless phone that allowed the connection of an external antenna to the base unit for use outside. I'm not certain as to the current legality, or even availability of this sort of thing (with a DECT set) as power output is very highly regulated, and, of course, the handsets output is rather limited, for safety reasons.
Most DECT sets, on the same frequency, consumer or commercial, should have, pretty close to, the same range from a single base unit. The internal antenna design will have an influence, but probably not much more than the surroundings, when used indoors.