NGK has discontinued the NGK 96987 SILZKBR8E8S spark plugs and they have been replaced by the SILZKBR8D8S (97506). That is probably why you can find the newer "revision E" plug so cheap at around 1/2 the price of the older "revision D" plug. It's been stated in many threads to use the SILZKBR8D8S plugs if you want one step colder. People have reported poor results with the 96987 plugs. If you want two steps colder, you want the NGK SILZKBR9F8S - (BMW# 12120042724).
https://www.ngk.com/product.aspx?zpid=40569
What is the difference between N20 plugs labeled SILZKBR8E8S (96987) and SILZKBR8D8S (97506)? According to NGK, the codes tell us that the discontinued 96987 plug has a different firing end construction that doesn't appear to have worked too well.
http://www.sparkplugs.com/Data/uploads/Charts/NGK_Plug_Chart_2.jpg
I run the N20 1-step colder plugs with NGK part number: SILZBR8D8S (97506). I gaped them to .026." I've replaced them 2x already and both times they came out looking to be operating perfectly on a hard driven car. I chose .026" because it's somewhat of a middle ground between gaping down by .010" to improve performance and leaving the gap stock for idle and economy. I might try going 2-steps colder because the plugs look to run a bit on the hot side still.
I also run the N55 eldor coils. They are supposed to be an improvement in design over the stock Bosch coils which started to misfire at around 100k miles. The Eldor coils have a metal band around them for cooling? Seems to work great and they are cheaper.
I have never had any problem running plugs at .026" with as much as 24psi at low rpm on the stock turbo. I now have a hybrid (PS1 equivalent) in the car and still don't have any issues. I am not too sure why N55, and even most N54, owners think they need to gap plugs down. Probably has something to do with early days of N54 tunign and JB4 only tuning where owners (and their tuners) thought it was OK to run 1300psi rail pressure. Then blamed park plug gap for the resulting misfires LOL
BMW forums is the only place I can really find anyone relating timing corrections to spark plugs... Timing corrections are the result of knock. Bad spark plugs usually result in misfires. Changing spark plug gap has such a minute affect on ignition advance that it doesn't make any sense for it to have a meaningful impact on timing corrections and knock.
People gap plugs down to prevent spark blowout (misfires) in high compression applications. They don't do it to fix timing corrections (knock). And, people run colder temp spark plugs when making more power to keep the spark plugs from melting or glowing causing pre-ignition. Due to the higher in cylinder temperatures and quicker acceleration the spark plug has less time to shed heat and needs to be 1 or 2 steps colder (which is a rating of how quickly the spark tip sheds heat).
The only caveat to the above is that a colder plug that idles a lot may foul out if you don't run your car hard. Again, this wouldn't be the spark gap causing timing corrections (knock) but rather a too-cold spark plug causing carbon buildup.