I think I understand what you are asking, correct me if I'm wrong.
You have a UHF radio.
You want to be able to transmit and receive farther.
You are asking if you used a better antenna in place of the stock one, would it greatly improve coverage.
Does that sound right?
So, short answer YES.
There could be some legality issues here. If you are using an FRS or GMRS/FRS radio, then removing the antenna would run afoul of the FCC. FCC says FRS radios that have been tampered with are no longer approved for use. Removing the antenna would be considered tampering.
If it's not an FRS radio, or maybe it is...
The stock flexible antennas are usually poor radiators of RF power. They are designed for convenience first, function second.
If your radio has a removable antenna, then removing it and installing a better external antenna would GREATLY improve your coverage. I wouldn't venture how much, because there are too many variables.
If the radio has a removable antenna, then you can remove it and if you know the connector type, it's usually pretty easy to find adapters. Using the adapter and some coaxial cable, you can connect to an external antenna and get some great results.
There are a couple of things here:
The antenna needs to be specifically designed for the frequencies you are transmitting on. As a tuning fork vibrates at a specific audio frequency, or as a piano wire makes a specific note, an antenna will work best when it is a specific length. There is some math involved here that I'll bypass, but using a random length of copper wire isn't going to necessarily work well. A properly "tuned" antenna will allow your radio to transmit with proper efficiency.
Antenna height is the other thing. The higher you can get your antenna up, the farther it will "see". Since UHF tends to work line of sight, the more it can "see", the better it's going to work.
So, find the correct adapter for your radio. Get some decent coaxial cable and a proper antenna. Get the antenna up high as you can and you will see a lot better coverage, both transmitting and receiving.