I checked Rock Auto
Both Aisin - the standard and economy - have no specs on the kit regarding the metal impeller or plastic, both kits carry a 12 month/12000 mile warranty
The Continental/Conti are listed as premium ; their specs say plastic impeller but carry a 24 month/24000 mile warranty

 

I bought the Aisin kit and I'll have to check when I get home this afternoon, but I could swear that the impeller is metal on the new pump..

 

When I compare the TKT-021 description to what actually came out of my car (original from factory, pump stamped with date of 7/05), it is very close. My idler was Koyo, as is the kit. The kit lacks information on the tensioner, which was NTN out of the car. The tensioner bearing was NSK. Belt had no info out of the car (likely markings worn), but Mitsoboshi is supposed to be the gold standard?

What could be different is the kit might be made in USA (actually I think the wording is "assembled", and Lexus original from Japan, and likely the Lexus kit....seems like so many people used the Aisin kit and have had no issues...I thought the impeller was plastic...