Here's my take on valve seats.

1. Yes, the factory cut the stock valve seats with general, 3-angle face out of necessity. This because it is necessary to clear the port and the combustion chamber of excess seat material. The 45° seating face is left generously wide for engine reliability.
2. For simple road riding, single angle 45° valve seats are all that's needed. So the faces you see inside your combustion chamber are generally what the factory planned.
3. 98% of valve seat cutting is necessary only after replacing valve guides. The other 2% is caused by excessive valve recession, and in most of those cases, installing new valve seats is the better option. And even when cutting is required, the cutting stone only "kisses" the seat for a micro-second.
4. Yes, it is true, there is horsepower there in the valve seat. But not enough for any road rider to concern themselves with, and proabably not 98% of AHRMA racers either. 
5. Spending money on a "3 angle valve job" is useless without the flow porting to prove the right angles were utilized. So if you have a local shop touting their valve cutting services, then look around for a flow bench. If you don't see one, then hand them the $500 and get the sticker to put on your bike because it's simply bragging rights.

I worked (still go in some to help) my friend who is an AMA Expert rider for 40 years, who is AHRMA No1 in his class, who has road raced at the Isle of Man and Barber, done speed trails at Bonneville salt flats, and still rides the high banks at Daytona and Talladega annually. He considers 3-angle valve seats a waste of time. 

@wobbly thanks for your input.

No racing for me, doubtful I'll ever ride over 70mph.

These valve seats look decent and passed the leak test suggested in a previous post. However the stems are sloppy in the guides so I'm thinking of replacing the valves, springs, and guides. Can the new valves be lapped into the seats by hand or is it necessary to recut the seats with the new guides pressed in?

1. We don't replace ANY engine part by "feel". Cold hard numbers are the only signal. With those we can determine: 1. Is there too much clearance? and 2. Is it the guide or the valve stem or both that's worn?

2. If the heads are properly heated and the new guides pressed into place, just a simple kiss with a 45° stone is all it will take to get the old valves (trued on a valve grinder) to "lap in". 

3. The need for seat cutting is because the new guide almost never goes back where the first one was. Think about it, just an angular change of 0.01° will throw the location of the seat off. The cutter is guided by a precision shaft inserted into the guide, so this work cannot be started until the new guide is seated in place and finished to size. (Sometimes new guides need reaming.)

4. The valves should always be trued at both ends. You'll hardly ever find a problem, but it insures they'll "lap in" right away. And it cleans up all the pitting from ingested debris. 

5. New springs are optional, unless you are racing.

6. I have heard nothing but horror stories about Black Diamond valves. The only "good thing" is that they are cheap.

7. BMW valves and guides are very expensive, which is one reason we measure. Which takes us back to item #1 !

Hope this helps.

@wobbly Thank you, this was most helpful.