The Class 800s will most certainly be entering passenger service with uprated engine power. With the uprated values, they top out at about 118-120mph, which is just sufficient to keep the HST sectional running times. However I predict they will not quite keep time on the uphill sections, as has been said they seem slightly sluggish uphill. My personal experience was 92mph sustained climbing from Bristol Parkway to Chipping Sodbury, where you'd expect an HST to be comfortably over 100mph. In practice this will drop perhaps a minute against HST schedule but it will be made up by Wootton Bassett Jn, so no real problem. Where I think the train will struggle to keep time is through Box on the Up. My prediction is 1-2 mins net loss against timetable but we will see. Remember the impact of a full load of passengers will change matters. Weirdly it might favour the 800 over the HST because of the bizarre way in which power is managed by the computer.