Saying that though, didn't MML used to run the semi fast with 170s which similarly couldn't have been too many minutes slower than a Meridian at 110mph.
They still found justification to provide high speed stock for these services.
I think the Turbostars were quite a bit slower actually - for example Leicester to Loughborough was 13 minutes on a 170 and is now 10 minutes on a Meridian.
The provision of high speed stock was justified because the semi fast services didn't need to layover in Leicester with the Meridians (so freeing up track & platform capacity in the Leicester area), because of their high speed and faster acceleration (so they could keep out of the way of the HST fast services more easily.)
Also, the half hourly service to Wellingborough, Kettering, Market Harborough & Loughborough provided by the Turbostars had caused passenger numbers to grow from these stations to the extent that Midland Mainline had to do something about it otherwise there would have been a serious overcrowding problem on MML's trains.
And last but not least the stakeholders and businessmen of Nottingham and Derby wanted faster trains.