On the course, did you then all move back after it started planing? - there's plenty of threads in the SIB section about that.....
For what it's worth my old SR4 had 2 30L fuel tanks, the front tank had a hose to the engine so I didnlt need to lug a 30 KG lump across the boat at an inaoppropriate moment. I found that if I emptied the rear tank first, as you say, time to plane was reduced (slightly), but if I emptied the front first, the helm became lighter, and the top speed improved by about half a knot..... but it took forever to get on the plane! I eneded up deciding on which tank to empty first by what I was doing with it at the time. I had a fixed trim 25 on the transom, no idea what you are shoving your Valiant along with, but the effects I saw can be accounted for in real time with a PT on the engine.
If you open the rear up for more space, is it for stowing more stuff? If so you are likely to alter performace more by loading the boat than moving you forward 18" on a loaded boat. If however you are single handed or you+ crew on a dingy club rescue boat and need the space for that then moving the the engine trim pin (or your trim switch if it's PT) will probably negate any effecs of moving the console on a light setup with a singl tank of fuel.
If you want to experiment, a couple of sand bags or a portable tankat the bow will proably give you a similar performance effect.