Most modern portable radios, regardless of manufacturer, are what they are. Because of micro sized components and multiple layer pc boards, it's no longer easy to modify any part of the radio except for maybe a mic connector or speaker plug. So increasing the wattage of the radio itself, while theoretically possible I suppose, is just not doable for 99.999% of us.
You could look into improving the antenna system. If you're in a vehicle, there are antenna adapters that screw on in place of the flex antenna so you can connect to a vehicle mounted antenna.
There may be some linneys available, but why bother? For the cost and effort, you can buy several 25 to 50 watt mobiles.