Are they that expensive now? I recall the HGT's being about 2k per gear. Planned to slap one onto a K20 swap in a Miata in the future (distance unknown).
I think that was all in, bellhousing, shifter, clutch, and a guess on driveshaft cost. They're not cheap, but you're not going to get that level of performance for the weight anywhere else.
Look up ASMotorsports, they run One Lap with a K24 swapped S2000, and do VERY well with it. They run the BMW DCT, and I was very on the fence between them. The gearing on the DCT is better, but after talking to Andy with ASM, I was more inclined to go with the HGT. Like I said, it's very much not a plug and play affair with the DCT. Now if someone were to come up with a truly drop in setup for JZ powered cars, it would likely still cost 10-12k, if not more.
The actual transmission from HGT is roughly 2k per gear still, but the other stuff adds up.
That's why I want to go the BMW DCT and get my speed set through gearing and not power. My supra is being built for autocross/ time attack/ endurance blah blah blah lol. But yes even though the DCT are heavy bastards I think its worth the weight penalty and reliability. And since I'm planning on going 1.5way LSD rear end my biggest thing is constant abuse reliability.
Good thing about transmission weight, it's actually not the worst place to add weight. It's low, and well between the axles.
Good plan to get your speed through gearing and not just power. Look at modern cars, even 200-300 hp cars are rather silly these days, and it's because of things like direct injection, responsive turbos, high compression, but more than any of that... gearing. Keep an engine in its happy spot, and it will be a rewarding experience.
Reason I suggested the HGT is that you will be up and running far before you ever would be with the DCT. Plan your differential around running a particular transmission and you'll come out ahead. I did that for my R154, and went with a 4.10 ratio. Coincidentally I ended up with the perfect setup for a T56 Magnum as well. HGT, a bit less so, as its sixth gear only has a 0.88 ratio for overdrive options. Cruising at 4000+ rpm is fun, but it wears you out on the highway.
OS Giken that rear end, by the way... just trust me on this one.
Any sequential or dog engagement box will need considerably more maintence then a synchro box due to the design of how the gears engage, regardless of the power. You as a driver need to learn to drive the box in a way that wont damage it prematurely, and ECU tune will need to on point to manage the torque on the shifts as well if you do clutchless shifting. Lots of power makes the previous points even more important.
Those types of boxes are awesome when they are working and really change the dynamic of the car, but make sure you have a budget set aside for servicing on them, also need to consider your downtime if you have to send box off or get spare parts in etc.
Everyone wants to do racecar shit until its time to do race car shit comes to mind.
It really wasn't that bad. When I was talking with Richard of HGT, and let him know what my typical use case was (couple thousand street miles per year, and as much autocross and road course as our short seasons would allow) he recommended that I drop the transmission for an inspection and fluid change every two years. Said he didn't expect it to require any rebuilding given that I was well within its operating window.
Driving it properly and having your ecu tuned to work with it is important, you are right. I believe it was closed loop operation that I was asked about when I was looking into it. Basically the strain gauge on the shifter sends a minor voltage to the ecu, as soon as you put force on that shifter, the ecu cuts torque output so the engagement on the next gear is softened.
As for budgeting, yes, yes a thousand times. Always have spare budget set aside for cars like this... but as for downtime, isn't that what determines a Supra's worth?