Unless the system uses a protocol that imposes a large transmission overhead, e.g. ACK/NAK, etc, keeping up shouldn't be a problem, since link-idle time on an asynchronous link shouldn't matter.

However, modem rates tend to favor a different set of rates than the old, tried-and-true rates (19200, and its close relatives) popularly used with terminals, though modems do certainly support them. How modem-to-modem communication happens is not relevant, since the issue is how to talk to a modem.

Most modems make it clear what rates are supported in their documentation. If one has that list supported in a setup table, successively trying them in order of preference, given that the modem supports the AT command set, one should encounter little difficulty in finding the most favorable rate that will work. IF the order is selected appropriately, one should not be saddled with an "unfavorable" rate if the modem adjusts to the rate at which it receives its AT message header.

Switching to a different MCU shouldn't be necessary.

The real problem with very high rates, from where I sit, is that RS232 was not designed for use at high rates. This introduces risk of encountering problems caused by the high rate that are not well-dealt-with in the EIA standard. Modern drivers provide some help, and careful cable design does too. Modem makers are motivated by marketing departments, which encourage them to support features that will "sell" while engineers often respond to that encouragement by providing features because they "can" rather than because they must. Keeping that in mind may reduce the difficulty in solving this problem.

RE