This answer to #2 in your question is, unfortunately, "maybe". The CM7r2 HBOOT (along with several others) is maintained by AlphaRev, and they note in their website's FAQ:
Will this touch my currently running ROM?
We try to leave the current ROM and datastructures completely intact.
It is, however, advised to ALWAYS nandroid backup your phone before
you run this procedure. You will then always have a correct/current
backup to fall back on in case something breaks.
So it's designed to flash without affecting your current ROM install, but it's a bit of a delicate process so there are no guarantees. There is generally always a certain amount of risk involved in these types of things, but especially so when dealing with the bootloader/HBOOT. Make a backup. If you can, copy the backup onto another device (your computer, for instance). Always prepare for the worst and hope for the best when it comes to data that you don't want to lose and you'll generally be fine.
As per your question about whether or not it's worth it - this sort of depends. The reason that some ROMs support certain versions of HBOOT and not others is because the different HBOOTs are designed to create different partition layouts on your device. There is a table on the website which details the layouts, and which I'll summarize here:
Name /system /cache /data
---------------------------------------------------
Bravo Stock 250M 40M 147M
Bravo N1Table 145M 95M 197M
Bravo Data++ 180M 5M 252M
Bravo Sense 180M 40M 217M
Bravo Oxygen r2 100M 5M 332M
Bravo CM7 r2 145M 5M 287M
Certain ROMs are designed to take advantage of the differing partition layouts, making them incompatible with others. For example, a ROM with a lot of stock components and additional packages may need more than 145MB of space for its /system partition, so flashing it on the CM7r2 HBOOT would not work. It's generally best to stick with whatever the ROM author recommends, which does mean that switching may be necessary in some cases.
