2866 Arent a lift spring, they are a light dual rate. With your weight, probably taken up the soft rate and sitting all in the firm rate. A taller spring and/or airbags for the weight variation. I had a similar rear setup and run 2863J...
Its never intended to be a 40 replacement, quite different vehicle. Additionally, the 40 replacement was made in 1984, still in production and in LHD.
https://www.toyota.ae/en/new-cars/lc70-hardtop/4.0-l/
I'm not aware of a rotor size upgrade, but definitely the larger calipers/pads are a help, not they should be matched to the master cylinder. I fitted HZJ75 calipers, which I believe are the same as 4Runner, DBA XP pads, but the biggest...
ARB OME MT64 now available for 100 series, fronts are extended length for longer travel. I changed to OME Upper control arms too as my previous Blackhawk arms maxed out on the upper ball joint travel. Available in heavy and medium tunes. I have...
Checking out the launch information a couple of accessories are available and able to be optioned from the dealer
https://toyota.jp/pages/contents/request/webcatalog/landcruiserfj/landcruiserfj_arb_acc_202605.pdf
Had them in installed in mine since September last year to replace my previous Blackhawk arms. Issue with the Blackhawk arms was the balljoint was binding at full droop. Changed to the ARB arms which allow more down travel and they are working...
You asked about improving performance? Leaks = Bad performance. Dont need to overcomplicate things, connect it up and go. Are you running out of vacuum now? Additional reservoirs wont improve performance unless your running out of vacuum...
My Dad's one had a chip, I did not tow with it, he liked it. The work one, last of the dual cab V8's, no chip, was disappointingly slow. Dad now has a 200 series, re-mapped with ~160kw at the wheels and it really moves along.
My Dad had a V8, i've driven it and plenty of other V8's through work. Most recently a current shape V8 with all the touring accessories (heavy), only with a 3'' exhaust. Sounded good, great offroad tractoring over the sand dunes with ease...