The stratos with the cap upgrades (known as a Stratos Plus) is a single transformer toroid type with 120,000 mfd. of total capacitance. The extreme has double the transformers for a dual mono setup and 180,000 mfd. of total capacitance with additional bypass capacitors (high grade) on each cap. The wiring (I believe) in the extreme is upgraded to Groneberg as well. The output capability is a little higher for the Extreme version (limited by the quantity and capacity of output transistors) as well as the amount of amps sourced from the power supply. In effect, it is almost as if you purchased two stratos amps running in mono, one for each channel. The only difference being the quantity of output transistors per channel, hence a lower output rating.
The overall improvement is really in the power supply. Cleaner and stiffer. (the toroids on the extreme are Plitron transformers higher rated....425 or 450va, at least as far as I remember)
The amp weighs approximately 74lbs. shipped in the extreme version vs. low-mid 60s for the plus.