Arguments over the need for nerfing of splash and speedbows aside, I dislike the idea of "Monotizing" existing cards.
I tend to agree with the school of thought that:
-nerfs to strategy types should generally be dealt with by addition of new cards
-Restriction / modification of the already narrow card set should only be done in flagrant cases (e.g. SoF)
The current card set is still quite small. Nerfing or removing a card has a dramatic impact on metagame. Heavy nerfing or removal of existing cards will reduce the number of viable strategies, which makes the game less fun.
To that end, why not implement the nerf to splash and bow use as creation of a card somewhere along the lines of:
(forgive the inelegant wording at the end, it needs work)
This will give a nerf to use of non-mono spells. It can be used in any deck, so it would have a broad impact on meta-game as a nerf toward speedbows and splash. But, it does so by making the use of low cost cards more risky rather than by forcefully modifying the cards themselves.
It will also allow the players to control the level of disruption. So a trio deck would pack maybe 1 or 2 of these in case of bows, while duos and monos could pack several.
Finally, rather than "Monotizing" the card, the nerf here scales proportionally to the number of quanta types being used.
This means bows (which seem to be the strongest offenders) will be the most heavily hit, while trios and duos (the lower grade "splash" offenders) are less strongly impacted.