The addition of an amplifier typically means the HT isn't as portable as the HT alone. It changes from a 'hand-held' to something like a 'bag-portable' type radio, or 'semi-portable'? Batteries for power, cabling, etc, that sort of thingy. If you're going to do that, then why not use a 'mobile' antenna with some sort of counterpoise? There would definitely be some advantage to a mobile antenna rather than a typical 'hand-held' antenna.
For instance, I hung a mobile antenna with a piece of string from a hook on the ceiling of my porch, and used a short'ish adaptor cable for that SO-239/SMA conversion to the HT. (Just fit so that the HT would sit on a table.) The counterpoise is a alligator clip with about 19 - 20 inches of wire attached, all clipped to the connector shield on the antenna.
The difference in performance was huge! Able to hit several repeaters that weren't possible with just the 'duck' on the HT. No amplifier was necessary, 5 watts was plenty. I know that's not what you probably had in mind, but it works. It shouldn't be all that difficult to adapt to a 'portable'/'walk around' type system.
- 'Doc
(The antenna is a dual band 2 meter/70 cm mobile antenna.)