Improved Firebolt reduced the casting time of your Imp's 
[Firebolt] spell by 0.25 seconds per talent point spent.
Notes
The imp's [Firebolt] has a base cast time of 2 seconds. With two points on this talent, the resulting cast time will be only 1.5 seconds. Combined with the talent 
[Improved Imp], the resulting Imp will be quite powerful and add considerable DPS. However, the value gets smaller for level 60 warlocks with epic equipment. The reason for this is that most epic equipment improves the Warlock moreso than the minion's, as the minion receives only a percentage of the bonus given to the warlock. Note that equipment that adds any type of magical damage improves the minion. The Imp's +spell damage will be a factor of whichever type of +spell damage the Warlock has the most of. (For example, if your Warlock has +300 spell damage across the board, but +350 shadow damage, the Imp will gain +spell damage based on the +350 from the Shadow Damage. If the Warlock somehow had +400 Holy Damage, the Imp would gain +spell damage based off of that. In the v2.0+ WOW client, this is revealed in the mouseover tooltip for spell damage.)
This talent only modifies the speed at which the imp does damage and with both points invested the imp will run out of mana very quickly indeed, which is usually less time than that needed to kill a mob. Although the imp has very little downtime, this talent will accentuate it slightly.
Tips
While this talent does not particularly favoring "leveling" side, since the imp will run out of mana quickly, this talent may aid in pushing back spells of enemy casters in PvP, or to keep enemy rogues in combat to prevent re-stealthing (not counting vanish) incase they blow Cloak of Shadows to remove all your dots. While imps can already do this without the talent, reducing the cast time makes them potentially more effective (0.5 seconds may be the life saving difference whether the enemy's spell goes off or not)
Patch changes
External links
Patch 3.0.2 (2008-10-14): 
[Improved Lash of Pain] and Improved Firebolt have been merged into one talent, 
