More than likely the problem is due to a harder pad surface. For some reason most aftermarket pads seem to be like this, especially performance pads that are supposedly designed to resist heat and wear better (if that ends up being true).
Here's something that's pretty interesting. Brake pads are made up of the following components, generally:
1. Asbestos (or Kevlar)
2. Graphite (to eliminate heat from dragging on the rotor)
3. Chopped up brass "steel wool" (for metallic pads)
4. Cashew nut resin (yes, cashew nuts! That is the agent that holds it all together)
The specific combination of additives in the pads makes the brakes harder or softer to a degree. If less friction can be achieved with a certain design, it requires more brake pressure to stop than other setups. This is not to mention the fact that since your rotors are drilled/slotted, you've got less friction surface stopping area to bring the car down to a stop, hence more pedal pressure is needed.