Welcome to the forum Kev. Lots of factors to consider here, including the current speaker placement. Can you share a photo of the placement so we can help you out? A 100w siren with 100w speaker should be sufficient.
Other factors to consider:
1. You can drive faster than the siren is projecting the sound out. Considering that you said this happens mainly on the highway this could be a very real possibility. I will try to find the study that mentions the approximate speed that a siren loses effectiveness because of this.
2. Perhaps more forward warning lighting is the actual solution you need to overcome number 1 above.
3. People are very distracted these days, cars are better insulated, and car stereos louder. You can't always overcome the human factor.