Call me crazy, but I like good metrics.
"Ran a bit better" and "much happier" are very bad metrics. They're completely subjective, and as such are prone to placebo effect to the point where I would generally completely ignore them. The metrics that you DO have are all indicating that the platinum tipped plugs are better.
I ran platinum plugs, as per stock in my 7M-GE. I daily drove it for half a million kilometers, in Canada, so cold starts were far more common than warm ones. Most time I didn't have the option to plug it in, either, so there were a lot of VERY cold starts. Never had a problem that related to the ignition. Battery, sure. Ignition, no.
Maintenance on the Supra wasn't skimped on, but I certainly didn't go looking to do more. If I had added removing the throttle body and Y pipe to the yearly task list, it would have been a stupid thing to do. Add in that I put on 465,000 km between Feb 1996 and Apr, 2007 and that's actually twice a year, thanks. As it was, I was kind of stretching the lifespan on the platinum spark plugs, since I went through 3 sets in that time. At a lifespan of 20,000 km, I'd be on my third set of coppers on the 7M-GTE that I swapped in. As is, I'm still on the same set of NGK platinum plugs that I installed with the engine. (1 heat range cooler than stock.) It's not about being cheap, it's about chances to mess things up by removing parts, opening up the cooling systems needlessly and replacing gaskets when it simply does not need to be done.
For performance... On an otherwise stock 7M-GE? Really? I hardly think that spark plugs are going to be any help whatsoever. Copper having less resistance when you're pushing a spark across an air gap is kind of like having 4AWG wire in a USB cable. I bet I could get a better performance gain by taking the burrs off of the distributor cap and rotor. The more durable platinum plug tip would give a more consistent spark as well if you properly shape the ground portion of the plug and gap it well, too.
I'm always skeptical when people make claims that changing something gives gains out of proportion with what I feel is reasonable. Performance gains from spark plugs? MAYBE when coupled with other changes, but on an otherwise stock 7M-GE? I just don't see it happening.
I'm not being cheap either. I own 2 Supras, I know what it costs to maintain them, and make them last. At the power levels that I'm planning, I do not feel that there is enough benefit to be had to make the additional maintenance of spark plugs that don't last worthwhile. I'd figure that a higher power ignition source would have a more reliable and durable effect, but that's just me.
For electrical resistance, yeah, it's true that platinum has about 6 times the resistance of copper. A quick back-of-the-envelope calculation gives me a difference within the spark plug of about 1.5*10^-6 ohms for copper, and 1.0*10-5 ohms for platinum. By comparison, a 1.8mm air-fuel gap under about 250PSI has an initial resistance of about 1.8*10^12 ohms. Once the spark starts, the plasma in the gap has a much lower resistance of only a few kohms. Clearly the reduction of resistance in the spark plug is unimportant. (A difference of a nanometer or so in the spark plug gap will more than swamp it.) The larger surface area of the copper electrode is probably the more significant factor, until the edges of the copper start to erode over the first few dozen kilometers, after which time, you have a constantly shifting variable, and the math starts to get ugly. I followed that down a couple of columns of numbers, but it's never enough of an advantage, and it's only once the average gap starts to increase that it really changes anything. Over the lifespan of the copper plug it goes from being very similar to the (similarly gapped) platinum plug in terms of overall power dissipation to being quite a bit worse, due to the gap lengthening. I assumed that a change in gap of 20% (ie, 1.8mm to 2.0mm) would be the lifespan of the plug. (I was calculating power dissipation since that is what I believe ignites the fuel.) This is partially decided by dwell time, but I was assuming that would be equal between the two different plug types.
For what it's worth, the parasitic inductance of the spark plug wires is of more detriment to the spark ignition than the material or even surface area of the electrodes themselves.
tl;dr. There's not enough performance gain to be worthwhile, just get stock replacement spark plugs.