Not at all. It's the efficiency of the speaker that determines how loud it is with a given input power. This is measured in dB at 1 Watt, 1 metre distance.
It's not uncommon for very powerful speakers to be very inefficient, so on a small amplifier, they actually sound quieter than a more efficient, lower power handling speaker.
There's no guarantee they'll be louder at all, but they should have a better frequency response
There's no guarantee the replacement speakers will sound louder, just because they claim to be 300 Watts. They should sound better, but that's very different to being louder.
The cheap JVC speakers I fitted to my FL1 were better, but they weren't louder for a given position on the volume control. Ultimately they had better clarity, simply because they had stiffer polymer cones, rather than the pulped paper cones of the factory speakers. This enabled them to provide more solid bass, but eventually the amplifier distortion covered up any gain in clarity.
The only true way to improve bass, is to install a sub, which I did, on 2 FL1s.
If you've been fitting ICE for 20 years, why did you say you'd struggle with 2 speaker wires.