Yes 2.25" is standard for the HM exit ID.
Whether you notice the difference depends on the tuning as most tunes are not really customised to the setup. What you notice is the trade-off 90% of the driving time when you want torque but seems less (doughy, laggy) when tuned for the larger diameter to make peak power up top. So to be clear you don't hardly feel 10-20 fwkw (15rwkw) screaming its tits off up top, but you certainly miss the wider torque spread that comes with higher velocity and scavenging low to mid.
FWIW I've never been an advocate for larger diameter short throw headers on our car, I did some calcs then dyno trials
took the choke off using varying diameter (hence tuning velocity) of the stock downpipe just after the header flange: 2.25" is ideal for HP and TQ balance with a (near) stock engine.
The two tonne wagon (all variants really), gear ratios, V8 and chassis character all responds better to more torque than unproductive screaming HP.
Unfortunately for most owners (at least on Facebook) there is one number to describe all the detailed facets of engineering and factors affecting state of tune, that being (estimated) peak engine power.
BTW I posted results of investigation into LS3 pipe lengths and diameters using PipeMax and Engine Analyser Pro somewhere here will link if I can.
In other words yes there is a trade-off to get more peak power.