Yeah, pretty much dead on. A boost converter increases voltage, a buck converter reduces it (just convention). So yeah, A boost converter set at 200V will draw double the current from your 100V pack (and a buck converter at 100V would draw half the output current from a 200V pack). You can get DC-DC converters that can supply a voltage higher or lower than source (usually a boost converter feeding a buck converter), and an some topologies are bidirectional (act as a boost converter in one direction and a buck in the other) - the Prius converters are bidirectional (to allow battery charging/regen), the gen3 is a boost/buck too!
Bear in mind that the prius/gs450 are not electric vehicles - The design is really intended as an 'eCVT', with MG1 taking power from the ICE and using that to drive MG2 (advantages are that the ICE can run at its optimum rpm, and there's no gearbox as such. Disadvantages are the losses between MG1 and MG2 but these are pretty small in comparison to running the ICE away from it's optimum in most 'daily drive' situations). The battery is really only there as a small 'buffer', so they could get away with a lower voltage and the boost converter. This was all designed back in the dark ages of NiCad cells
That little boost converter can, however, dump 25-30kW into the drivetrain from the battery for a few seconds if the driver floors it, which is helpful!
Of course, more recent designs (including the Outlander and other hybrids/PHEVs) have seen the advantages of a bigger battery pack (a significant electric-only range, lithium cells have better energy and power density, so a 'full voltage' pack can still be a reasonable size
and drive a motor quite happily etc etc) so that's how things are moving. I believe the later generation prii have a more capable boost(/buck) converter, but the pack is still relatively (compared to full EVs) small, so will get warm quite quickly if drawing lots of power from them (on the plus side, with a small pack, it'll go flat quite quickly too!). The Outlander operates on 280V(???) directly (no boost converter) and is designed around that voltage...
There are other ways of getting high enough motor speeds. You can change the gearing between motor and wheels (which will require more torque, hence more amps, but the
power requirements stay about the same), you can change the construction of the motor (higher kV, rpm/volts - different windings, magnets etc), or you can do some magic in the inverter called 'field weakening', which changes the kV of the motor dynamically by using some current to counteract some of the magnetic field. This is definitely witchcraft
but does require additional current (generally, at full throttle, you would run an inverter 'flat out' at max current, so you'd end up with constant torque from 0 up to the 'base speed' of the motor, and then as field weakening kicks in, get a 'constant power' region where torque drops with speed, until max rpm. Of course, that fits in quite nicely with how ICE vehicles operate (acceleration reduces with speed, mostly due to gearing) so that's not necessarily a downside, and you get the full torque of the motor at low speeds where it's more useful (increasing the kV of a motor reduces the kT - torque per amp. If you increase kV mechanically, you lose that torque all the way through the rev range. Do it with field weakening, you keep full torque at low speeds!)