Here are my opinions on the questions you posed:
plaur, on 2012-March-13, 07:45, said:
How much is our hand worth and what do you bid? Is it close?
It is worth a heck of a lot more than 15 HCP would suggest. You have 6-4 (powerful) shape, a great suit, and not a single wasted honor. It is close, but the choice is between 3
♥ and 4
♥. I would bid 4
♥ personally. But if partner is good enough to realize that K
♥ is worth gold, and that honors outside of spades are much more helpful than ones in spades, then 3
♥ is enough.
I like to use the concept of "perfect minimum" here. On the auction, assume the 2
♠ bidder has about 9 HCP. We have 15, so there are 16 remaining in the deck for partner and LHO. A normal assumption in competing over a preempt is that partner has about 8 HCP, which in this scenario is half of the remaining HCP. Let's say partner is a bit below that with 6 HCP, but has the perfect 6 as given by Art above. You would want to be in game, wouldn't you? Then bid it if partner is unlikely to know how good his perfect 6 is, and bid 3
♥ otherwise.
I realize partner's true minimum is 0 HCP, but playing like that is just scared bridge --> especially at these colors.
plaur, on 2012-March-13, 07:45, said:
Does form of scoring matter?
Yes. I'd bid 3
♥ at Matchpoints and 4
♥ at other forms.
plaur, on 2012-March-13, 07:45, said:
Does it make a difference whether you partner has agreed to play lebensohl? (this is BI !)
As Art pointed out, Lebensohl is not relevant on the auction given so far.