Okay, I think this is deserved for a number of reasons. It is powerful, versatile and decently cheap. I feel this is a lot for a card. Also this is often the most frustrating card in the dreaded Firestall, of which I feel will weaken a bit if this nerf is passed. Here is what I propose:
It basically loses lethality, but it makes the creature as fragile, yet powerful, as you would see a typical Fire creature. It still has potential in Firestall, as it can be synergized with Fire Shield | Fire Buckler for the final blow, but it requires an attack to work, making it a little risky.
Just in case wording is a little confusing, some examples:
Sapphire Charger (4|5) with normal effects of Rage Potion = 9|0 (killed)
Sapphire Charger (4|5) with suggested effects of Rage Potion = 8|1
Colossal Dragon (7|15) with normal effects of Rage Potion = 12|10
Colossal Dragon (7|15) with suggested effects of Rage Potion = 12|10 (same potential)
I do think that this card should have direct CC ability (i.e. be able to kill outright). However, I think a good way to differentiate it from other CC type cards would be to have the targeted creature not die until the end of its owner's next turn. I.e. the creature becomes reduced to the equivalent of a high power spark. It will have 0 hitpoints but will not die until it attacks, as would a spark.
I'm not sure Rage Potion was ever supposed to be used as much as CC as it is in its current form. 95% of its competitive use is a CC and not as a buff.
I had suggested this before:
The idea of reducing the creature to 0 hitpoints helps make this card truer to its original intent. You gain an enhanced attack for a creature, but at a cost. If the creature takes more than its hp worth of damage, it still gets one shot at using its extra damage potential. Conversely, if used as CC, it will still be as effective as lightning (or more) in terms of damage to target, but will require its user to weigh the cost to benifit before using... it becomes: is it worth taking a one time shot of extra damage in exchange for killing a potentially threatening creature.
If we retain the other suggestion from Malebolgia, and only allow the creature to gain power equal to life lost, this will prevent it from being abused on, say, photons.
Finally, since the targeted creature will not die till end of turn, this will open up other avenues for use that were not present before, such as being able to buff any creature by comboing with heal cards. Again, by limiting attack bonus to hitpoints lost, the potential for abuse should be mitigated to a good extent.
This change will make the card more valuable overall, but also prevent it from being "just another fire CC card"
Those are my thoughts on the issue.