Automakers are putting a premium on decreasing unsprung weight. Meaning anything not supported by the springs, including spindles, axles and wheels. Reducing unsprung weight improves handling by allowing the tires to stay in contact with uneven road surfaces due to a decrease in inertial forces. This can allow automakers to use softer shocks and springs to improve ride comfort without compromising handling.
Weight reduction also equals better fuel economy, but less weight means that the suspension has less mass to absorb vibrations from the tire/wheel assembly. Add to this that the tire sidewall is reduced to compensate for the increased wheel diameter and you have a trend back towards the covered wagon and the Flintstones' mobile.
Having said all that, I do not believe that factory size tire and wheel assemblies on the LS should be all that difficult to properly dignose and service. Your theory is right on the money.
if you had all possible components balanced, you still feel vibration, then it is driveshaft that someone mentioned earlier. weight came off from driveshaft.
I had same vibration felt at around 45 mile/perhour and then i had had wheels balanced by discounttire several times within a month and till felt vibratio. Later I had it checked by lexus dealer - test drive. According to tech, if it was wheel balance issue, then the steering wheels will be vibrated left/right, and he said my one was bibrated vertically along steering column, so he added that it could be driveshaft issue.
Following Lexus' tech hit, I checked driveshaft, I found there was a 20mm x 20mm rust area on rear part of driveshaft. It is the biggest weight I can find; all other weights are smaller in size. I don't know what is the thicknes of the weight, so I assume it wa about 1mm thick and made one and then attached to the rust area duct tape secured.
the bibration (actually it is called resonace) shiftted to different speed with different weights that I had tried. I could not get a right size (weight) but it is better than before (was like driving a train).
Tip:
driveshaft is heat treated so it is impossible to make a hole for screw. Also I used liquidnail and the weight came off next day. To use duct tape, worked just fine but to make sure it is wrapped in a right direction. o/w it will come off soon.
I also tied to use hoseclamp and it does not work may be too much weight added
I checked on eBay, one used one might cost $70, but i still leave it as is with my own weight/duct tape, as it is much better than before
I also checked local driveshaft shop - the rough estimattion is about $240 and they said they have never done with Lexus so don't knwo what is the outcome...