I'm just the newbie here but I couldn't resist throwing in my .02. First, all this talk about twins improving lag is not accurate. Compounds improve lag because one turbo (smaller) feeds the other (larger). You can google search to see what others are using or recommend but IMHO there are too many keyboard mechanics out there that don't know their ass from a hole in the ground. As for losing PSI from tailpipe turbo, I think 1-2PSI was quoted, you'll lose that thru an intercooler anyway.
There are advantages to MAF and MAP sensors respectively. I prefer the MAF as I think most tuners do because it more accurately reads the incoming air than the pressure inside the intake. As most have said, I believe the only way to tune is by the seat of the pants. Buying a mail order tune may or may not just get you running but having a real tuner log and read data to make adjustments is the only way to go or your just throwing your money out the window. You need to take so many things into consideration with tuning. What may work for someone out west may not work for someone like you in FL. Now you may get a data logger and you can get a tuner to view the data, make changes and then email you the tune. But don't spend $400+ on a one time deal.
As far as turbo or compound turbos are concerned, there are a lot of variables to consider, not just the turbos themselves, but the EH sizes, the turbine sizes, wastegate sizes, cold pipe size, hot pipe size and material used, downpipe size, BOV's etc. Any and all of these has an effect to one degree or another. Some not so much, others are critical. But to keep it simple, mismatching the turbos will limit the overall effectiveness of the setup. For instance, too small a top turbo matched to a very large primary: the primary will never really spool to get to its efficiency range, the small turbo can't effectively "swallow" the amount of air being fed and you'll likely end up with to high of drive pressure from the restriction. You'll get fast spool on the top turbo but suffer top end performance. Reverse the scenario: too large a top turbo results in laggy performance and the bottom turbo will spool in a fashion similar to the top... almost a wasted effort. A rule of thumb, that I can see, is the top turbo should put out in lbs/min roughly 50-60% of what the bottom turbo will be capable of. So using a he351 putting out about 55 lbs/min of air max, a good choice to pair with it will be a turbo that produces somewhere between 90-100 lbs/min for quickspooling towing application (think s474 or 475) but no more than 110 lbs/min for racing and that is overkill. The HE351 limitations being a small EH, turbine and wastegate limiting power potential due to excessive DP above the 600 hp range.
If just a single hair dryer is in your sights things get a little easier. You really need to look at the mapping of the turbo, know how to read it and get yourself in the efficiency islands of the mapping to get the most out of a turbo. There's a lot of math involved. But as long as you follow along its not that complicated. Things that need to be figured out are volumetric flow rate, efficiency, mass flow rate, pressure ratio, compressor temp, intake temp, density ratio and CFM of the engine before you can convert all that info over to a turbo map to see where you need to be according to the islands of the maps.
I find thermodynamics very interesting and love math so sourcing the correct turbo based on a particular engine is a nice challenge for me. I can share some basic info that could shed some light on the fundamentals for you or you can just "Google" it and roll the dice.
