Your radios need a specific amount of voltage to operate and maintain uptime. If it were a device with higher voltage requirements, a 1.2 volt fluctuation may not be that big a deal, but for a low voltage device like this, it’s a 20% drop in battery power, which is significant.
Will the radio respond with only 4.8v on RX? I don’t know, I’ve never tried it. I have a pair of GXT1050 radios but I only use the Midland rechargeable battery packs or alkalines, so I can’t say. However, it definitely won’t TX well, if at all, because that’s when the radio draws most of its power. The AVP7 (BATT-5R) rechargeable and 4 AA alkalines each supply 6 watts, and when the voltage drops considerably below that, the radio won’t have enough power to sustain operation, especially on TX. You will need a fully charged battery at full power for optimal operation during transmission.
Also, considering these are new radios, keep in mind that the use of third party rechargeable batteries are not supported by Midland and will void the warranty if they damage the radios. Midland offers a three year warranty on the GXT1000, which is very good for any consumer item, and adds value to the radio, so potentially throwing that away is at your own risk.
The reason these rechargeable battery packs are not specifically designed for charging outside the radio is because they are consumer products. Many business and industrial grade radios have battery packs that can be dropped in a charger on their own because the radios are designed for long duty cycles and multiple working shifts. The batteries for these radios are designed to be easily swappable so they can be recharged while the radio is still in use.
Most consumers do not operate their radios with such long duty cycles and do not typically swap out batteries between uses. They usually prefer the convenience of simply dropping the radios in a charger or plugging them into a USB port like any other mobile device when they are not in use or at the end of the day.