I beleive the sleeve bearing (is it not a fluid bearing as it does not make contact with anything but oil) not laggier in the respect most people state.

Lag is used by most people as a term that includes both the minimum rpm to produce boost, and the time from opening throttle to boost while in the rpm range of the turbo.

Now, i haven't driven a roller bearing turbo'd car, but i see it like this. When on full throttle from low in the rpm range, the difference in when the turbo will boost between roller and non roller is negligable - as the volume of gas needed to drive the turbo is not much different between the two.

However, from a lag point of view - what u are waiting for is the time taken for the turbo to accelerate to 100k+ rpm when more than sufficient energy is available to drive the turbo at that speed. This is where the roller bearing has an advantage. BUT - this advantage becomes less as turbo size increaces - due to the inertial becoming the majot factor, to the point that when you get to T4 size the difference between roller and non roller is marginal.