The Brit and Clan share similar chassis, but the boiler was smaller on the Clan. I think a lot of the "problems" with the Clans was that because they were
4-6-2's - they were perceived as "powerful" locos, whereas they were designed to be a reasonably sized loco with low axle weights. Maybe a classification of 5 1/2 was probably better than a 6! Typical for all pacifics, they required careful handling to avoid slipping under starting. 4-6-0's have an inherent advantage when starting as the weight settles on the rear driven axle.DJH make a kit for a Clan.
I suppose the modern comparison is the Class 66, compared to say a 59 or 60. The 66 is a "go-anywhere" loco with moderate pulling power, whereas the 59 and 60 are specifically for heavy haul duties. In the 66, heavy duty rated (and thus physically heavy) electrical transmission gear has been sacrificed in order to carry a large fuel tank (and hence greater operational range), when compared to the 59. The gear ratio on "standard" 66's is also optimised for 75mph running. Some sort of compromise to tractive effort has been made with the lower geared and slower
66/6's. It wouldn't surprise me, that when the 66's come round for major refurbishment, some are modified for heavier haul duty (as was done with the 37/7's for example.Cheers, Mick