By summer I want to upgrade my 70 qt mash cooler to a blichmann 20 gal w/false bottom which I currently use as my boil kettle now. My pre boil volume is usually 15 gal. When I whirlpool in my 20 gallon, the trub never really seems to settle in the middle . I know my immersion cooler isnt helping. Would the increased surface area of a 30 gal boil kettle greatly aid the whirlpool and get more trub in the center?

The aspect ratio (diameter to height of the liquid) will affect your whirlpool and the more that ratio leans toward the width, the better the cone. But doing the 15 gallons in the 30 gallon pot will increase your boil off rate quite a bit, which may end up being too high.

My boil off with the 20 gallons is 9%/hr . It isn't a violent boil, but it is kind of high. Even using natural gas @ 60000 BTU. I can only put 12 gallons into temp controlled fermentation.

I think you're well within the 'normal' range...

A brewer who I respect once told me that the ideal ratio of width to height for a whirlpool is 7 to 10. I've certainly found this to be pretty accurate. When I have 17 gallons of wort in my 26 gallon kettle at flameout I don't get near as good of a trub cone (as measured by the width of the ring of clear beer around the edge of the kettle) than when I have 12 gallons. Yes, you'll get a higher boil-off rate when doing a small amount in a big kettle but that's just a matter of adjusting your pre-boil volume to match the boil-off rate. All the popular software handles that nicely. Yes, an immersion chiller gets in the way during the whirlpool. That's another reason I love my external chiller.
Tasty

Another way of looking at this to not think in terms of rate and percent, but in volume/hour. For example, if you are boiling with the same flame on the same pot, you might be losing 1 g/hr, regardless of your volume (since you've got the same flame and same kettle geometry). If you were to only have 2 gallons in your kettle, you would have a different rate (not 9% per hour), resulting in a higher percent being lost per hour.

I'm assuming it will also vary based on the shape of your kettle (tuna can vs soda can).

It likely doesn't scale just based on volume/hour either... perhaps some sort of logarithmic type relationship? If I get ambitious I may play around with measurements, but the long and short of it is that boil off loss (as measured by % lost) is going to vary based on your starting volume. The rates put out there are based on a standard size batch, so when JZ for example gives his rate loss, it is based on the standard Jamil Show preboil batch size.

A major reason for trying to get a better trub pile is because I do want to go to an external plate cooler and I dont want to pass a lot of trub thru it. Does the wort exit the ball valve to a March pump to the external chiller then goes back into the boil pot? When the temp is where you want it ...into the fermenter? For my brewing a 20 gal boil pot is sufficient. My last question is about whirlpooling. I have been doing it starting at the flameout in conjunction with the start of the immersion cooler. I have seem some web site say to wait till the wort is chilled down, remove the immersion cooler, stir up a strong whirlpool, cover and wait 20 min. Can you clear this up for me!

This is exactly what I do. I chill the wort down to pitching temps with my immersion, take the chiller out of the pot, and then create the best whirlpool I can using my ss spoon, put the cover on the pot, and go do some clean up for 10 minutes. I usually have a good looking trub cone at the bottom and rack clear wort into the fermentor.