As you might expect from my username, I was (still am) an enthusiast of the 309s but am aware that no MKI EMUs stand direct comparison with even MKIII types as the power to weight ratios were much more modest in the '60s. The 321s had ratios of 7.7kW/tonne whereas the 309/2&3 class did their thing on just 4.9kW/tonne. They were also the only MKI EMUs geared for 100mph.* That is unless the REP 'tractor' units were counted, (12.5kW/tonne) because they never ran in passenger service without some deadweight 'TCs in tow. That would be like comparing the 309/1 2-car units in isolation (although they did often run alone between Thorpe le Soken and Walton-on-the-Naze), but that stretch of track was no speed-test strip. Their 8.4kW/tonne - would however have made them pretty snappy up to speed on the GEML.
* The 432s were geared for 90mph although there are many, (maybe apochryphal) reports -some on RUK, of wild speeds in excess of 100mph being logged. A 309 consist did actually acheive 109mph on some high speed test runs with frontal streamlining over a fast section of the WCML near Cheddington but their genuine 100mph gearing with less than 5kW/tonne and their 0.9mph/sec/sec rate was never going to make them rapid accelerators.
Interestingly, when the class 312s were introduced, their 90mph maximum speed was questioned, but it was explained that their faster acceleration (1.1mph/sec/sec) enabled them to slot into the faster ICs running both on the GEML, ECML and parts and the West Midlands LMR network, (much like the class 700's 100mph maximum is mitigated by their acceleration on the MML fasts).