I speak from the standpoint or a Gold 1 player who predominantly plays Irelia top. (#platlyfehereicome)
I believe feral flare as it stands is an entirely balanced item, and I'm not entirely convinced that your idea of buffing it would be bring greater balance to the game. Here's why. What I believe a lot of players don't understand is that the jungle is far more similar to being in a lane than people give it credit, although it may just be abstracted by the lack of direct competition and logistic consistency, with a few key differences.
In laning, both players compete in CS, objectives, and kills, which all translate to gold advantage. Jungle is similar except that, unlike laning, scoring a kill is not negatively correlated to time (and cs). If the jungle wants to possibly develop a kill advantage their cs will suffer as a result (and this deficit will rely on skill to some degree). That's what the jungle pays for for the utility of being to directly influence any area of the game.
At higher levels of play, this utility is the more focused on aspect of the jungler over the relative ability to safely farm. Ganking became the norm.
Feral Flame was originally introduced to give farm-focused, late game junglers more viability against teams that pick safe, early lanes, and to give gank-oriented champs and alternative in the same scenario. (Think LB, heimer, or Janna.) Frankly, Udyr sucked for the longest time because his only gank was running straight up to the laner with slightly higher than average movespeed. Feral Flame gave him some life.
However, should your buff be implemented, you reinstate the totally predominate gank oriented jungle meta because having the buff sit on machete and madred's at +2 stacks gives every single, gank-oriented jungle an insane snowball mechanic. If you look at the current meta, we see Vi, ww, j4, amumu, maokai, and elise, and all of them have extremely strong ganks. Now, if the buff exists on machete, early ganks can lead to a ridiculous snowball because the ganks pay for the wriggles, and by the time they have it complete, it'll be at maybe 20 stacks. At 12-15 mins the flare will be finished, and that's an insane power spike, all with no tangible tradeoff to the jungler.
And that's if they're successful in ganking, if not, then the forfeit in their build is nonexistent, because machete builds into every other jungle item. What would end up happening is AA oriented jungler's would be forced to sit on machete until their ganking powerspike (I'm thinking ww's 6) and then see how they perform after, while going into their core build in the meantime. If they do well, they build into feral with confidence, and if not, oh well. The trade is sort of there in jungle clear times, but it's not nearly high enough for the high reward potential.