In 2004 IE bought a BR Mark 3 TRFK for use on the Push Pull sets and it's the exactly same size, just got new bogies.
The reason our Mk3s are 100mph restricted are because of 1) our track standard 2) our signaling system and 3) 100mph is the 201's design limit well 105mph is. 071s have been known to exceed 90mph back in the day before they got the event recorders fitted to make up time when hauling Mk3s.
Same as the CAF Mk4, they could do 125mph if we had better heavier standard track, better block signaling and faster locos.