Hi,
I would say from experience that a good TT helmet DOES improve times compared to a normal road helmet. However, if I were racing against say Matt Bottrill wearing a road helmet and me in a TT helmet, I think I'd know who'd win. Matt..... everytime. What I'm trying to say is this. It's not down to the helmet. It's down to the rider, and your training. You could get another 30seconds off just by training without buying a TT helmet. Also, weather conditions affect your time. And you're bike fit. Do you use aero bars? They do make a big difference but only if you've got them and yourself set up correctly. Just putting on a set of clip-ons may not be the answer if you're not positioned correctly. It's the combination of a lot of things that will improve your time and get that long 23 you're after.
To answer the original question; "I was just trying to gauge the difference between with road helmet or without.
I am trying to get a long 23 in on my road bike before any additional expense on TT specific kit. I am riding 24's now so am getting close, just looking for some cheap aero advantage." I'd say this..... If you are getting 24's without TT specific I'd just consider tweaking your training to specifically achieving your TT aim. If you wanted to buy some TT kit, maybe start with clip-on aero bars. A TT helmet will improve your time but you'll need to get one that suits you so ask around and try some on before buying. Other things to consider are a skinsuit and overshoes/oversocks. Obviously, the type of bike you ride will have an effect on your time too.
Good luck and I hope you achieve your goal of a 23.