I have been working a lot myself, 4 hours sleep, not seeing the kids for days at a time etc. Hence the short reply.
The link is the guy who built my 4L60E something like 9 years ago. He built it when my goal was under 400 hp but for the last almost 7 years it has been 480 and crept up to something likely a little over 500, maybe 520fwhp.
He doesn't use a lot of fancy parts, and the shifts are not hard just quick. What he does do it take the time to set it up with all the right clearances with proven parts rather than the latest marketing gimmick.
When I began adding more power he suggested a few upgrades for me like sleeved input, maybe a billet servo or two instead of the stock "Vette" servo I have and I don't even have a Beast sunshell just stock.
Like I said the tranny was built with a milder goal in mind when the car was a 20K mile a year daily driver. He wouldn't have put it together quite like this had we known the direction the car would take but I am VERY happy with how it has held up given what I have done to it.
TH400 has a 2.48 I believe first gear compared to a 700R4 3.06 and also lacks OD and lockup, the lockup lets you use a performance high stall converter on the street comfortably.
I would never consider an old 3 speed tranny for a true driver. On top of that a TH400 is heavy and parasitic for a modest 500fwhp goal a TH350 would be plenty strong and faster than a TH400 since more power would make it to the tire.
T400 is tuff trans, but have to agree....not for a driver. RPM Transmissions does a good job as well on any of the GM lock-up's, but to hold much more than 600 or so HP the 4L80 (85) with a standalone controller may be the best bet.
Been awhile since I heard any current feedback on them but I know years ago they were briefly all the rage on the other Impala forums. Used to be pretty common over there someone would pop up with what they said was the best 4L60E sell a whole bunch then they all start to fail and everyone moves on. Careful assembly is critical. "upgrades" like the 5-pinion planetary are rarely needed on later model stuff, by the time the LT1s came out the 4 FORGED pinion planetary was quite stout, GM made a COST choice in 5 powdered metal pinions and the internet was all abuzz about this "upgrade". It might be a minor upgrade....................................................................to a part that really didn't need upgrading, but it IS CHEAPER for GM to manufacture.
I know some actually reputable vendors offer it as an upgrade I would like to talk to them "off the record" though and find out why that is, sometimes vendors have to offer upgrades they don't really like just to please an ignorant customer base.
The assy is always critical, and there are a ton of aftermarket hard parts for any of the 4L60e based trannies out there. I would recommend a few sources for him as there are a ton of sources not to use, and after the fact is to late.
I think the fanciest parts in my tranny are a cryo'd accumulator piston and a vacuum modulator. Input NOT sleeved, output NOT cyo'd, stock sunshell, stock planetaries, stock for an LT1 Vette servo. Lots of stock parts but careful assembly. If I had it to do over again for the current performance level I would sleeve the input, cryo the output and probably put an upgraded sunshell in but still wouldn't bother with planetaries and probably wouldn't bother with servo upgrades either.