You're right, kernels are device and rom specific. However, modems while device specific, are not rom specific. On other models, we switched out modems all the time to maximize throughput and coverage for our specific area. Just remember that an updated modem, while accomplishing something for the carrier, is not necessarily in your best interest, as sometimes they slow speeds or reduce coverage. That's why for most models we have modem archives to be able to switch them out.
Here's an example on XDA for a modem backup/switcher tool for the S3...
[Baseband] ALL T999, T999L, T999V, T999N Modems | Samsung Galaxy S3 | XDA Forum
So actually, it is quite easy to backup your old modem from MC1 and update to ND4 while leaving other parts of the rom intact, so you should be able to keep Sprintorious while still having the benefits of the new modem, if that speeds things up in your area. If not just reflash the MC1 modem...