I think of him as a transition guitarist, he bridges the 70s way of playing guitar and the 80s heavy guitar style, he is also one pioneers in sweep picking and fast movement of the pick up and down the neck,

Just listen to the solos of Hold the Line, Rockmaker, and You are the Flower from the debut album fast, melodic and precise sweep picking techniques.

On way I thing he style as it is are influential or fresh (back in 78), was that he's style incorporated three styles Jazz-chords, jazz style soloing, flamenco style, metal style, (fast precise), Holdworth, Larry Charlton, Jeff Beck styles are incorporated in he's style.

also in the 90s he was not death to how guitar was played then he learned both from grunge (he liked Soundgarden), and what grunge brought in guitar playing (hard heavy blues guitar riffs, repeated over and over in a groove, with a gruff style, Luke incorporated that as well in he's tool book ,

Jazz style, Latin style, blues/psych style, metal style and now the grunge style (so just as the 90s was in it childlike stage, Lukather was already in tune with the time (or even a bit of a pioneer even then),

What do you think of that? Is he relevant in the evolution of metalguitar technique

he's strongest album where he's guitar work is most pressent is on Turn Back, Isolation and Kingdom Desire, here you can hear he's technique is under a development, he's guitar work on Isolation album is blistering and very agressive and loud, fast, sweeping and great riffage, Kingdom of Desire is closest Toto comes to Alternative rock/metal, heavy guiar riffs, mixed with west-coast/AOR musicanship, Turn Back is a very intersting album, (bad production) but the songs are origenal and strange for Toto, three, four semi neo-prog tunes, semiconcept album.