The Nexus S and the Samsung Galaxy S (and its variants like the Captivate) are pretty much the same, with similar hardware except for a few differences. That’s probably the reason that ROMs ported over from the Nexus S to the Galaxy S work pretty good with a few changes.
The Galaxy Note i717 was recently hacked to work on T-Mobile’s HSPA+ network, which gave developers and users over at XDA something to think about. XDA modder pwneman went ahead and took a look at the Nexus S radios and found out that the modems/radios of the AT&T Nexus S i9020A are compatible and work on the Galaxy S and the Captivate. Not only that, using the Nexus S radios provides some advantages as well. The network strength seems to improve by a noticeable margin, apparently also improving call quality. HSPA speeds also saw an increase when tested.
The radios work well on the Galaxy S but on the Captivate, the 850 MHz band may not work (the radio band on which 3G works in the US and Canada), and connectivity may also be a little unstable. Also, the radios have been tested only on AOSP (MTD) based ROMs like CM7, MIUI and all the Ice Cream Sandwich ROMs, which use the radios in a different way than the stock Samsung ROMs.
As you can see, using those radios can provide tangible improvements, so head on over to the Galaxy S page and the Captivate page on XDA to find out more about the procedure and to download the radio packages. Again, keep in mind that these are only for ROMs like CM7, MIUI and ICS ROMs, so don’t try them on other ROMs. Let us know in the comments about how it works out for you, and whether you see improvements on your device.