The realistic progression packs give you a set of real world rocket engines and fuels which do give you a bit better ISP that you're used to in the main game. You'll still need much bigger craft though. Fortunately the procedural fuel tanks make this a lot easier and help reduce the part count too. Fuel Density becomes a factor too. Liquid hydrogen is very light but needs a lot of tank volume, Kerosene is denser so smaller tank size but less dV. once you get your head around the pros and cons of the three main fuel groups (kerosene/Liquid Ox, Hydrogen/liquid Ox, UDMH/IWFNA) its not too tricky. The main downside I'd have to say is the launch times. building up to an orbital velocity of 7500m/s takes a long time especialy when the high part count at launch is causing physics to slow the game right down.