One of the interesting things about Telling Blow, is that it is skewed towards weapons with a wide crit range, as there is no multiplier involved.
Multipliers _and_ crit ranges together make weapons balanced towards each other. 20/x3 and 19-20/x2 is considered equal. But with telling blow, you would never use an axe or pick - crit range determines all, so rapier/scimitar/falchion/kukri it is!
Good or bad? Telling blow would be a little too nice if the dice were multiplied.. sure, a pick would result in humongous damage, but I think in practice, it would be overkill in mony cases... in practice rapier would be more consistently good. Certainly too good for a feat, IMO..
An idea for a feat that preserves the crit range/crit mult issue, would be a feat that adds +1 damage for each sneak attack dice on a critical? Meaning that a 9th level Rogue (with a 12 Str) criticalling with a +2 light pick would deal (1d4 +1 +2 +5)x4 = 42 average damage.
Change the extra damage to +2 for each sneak attack die, and call the feat Improved Telling Blow, perhaps? A 9th lvl rogue would deal 20 extra damage on a crit with a rapier (+10 for the feat, x2) - with a battleaxe, +30 damage, and with a pick +40 damage.
Otherwise, I agree with the above posters.. as written, you just get one more situation where to apply the sneak attack dice.. no multiplications!