I have built a few fairly light kayaks designed with Kayak Foundry. I have some 15' sea kayaks that weigh just over 10kg and a 15' TK1 that weighs 8.3kg.

You could get it a bit lighter than that because I used 125gsm (4oz) glass inside and out. The sea kayaks are made from 5mm strips and the TK1 from 4mm. I used mostly Paulownia, which is about 20% lighter than WRC. I would not hesitate to use 4mm for any kayak used on inland lakes. You could get the weight down a bit by using 3mm strips and 80gsm glass.

My attitude is that the first step towards building a light kayak is to get rid of the unnecessary bits and keep it as small as practical. I have the advantage of weighing about 63kg, so if you weigh an extra 20 or 30kg, you would need a slightly bigger and therefore heavier kayak than I do.

Most people paddling on flat water don't gain much by going over 15' unless they are particularly energetic paddlers, so If you want a light kayak, that length is a good starting point. Keeping the width and the height of the kayak down also helps keep the weight down. The Black Pearl is a beautiful kayak, but would not be my first choice if light weight was the number one priority. That bow and stern looks great, but they add weight without helping the performance of the kayak. As I am sure you know, it is the waterline length that affect the speed, not the overall length. That is not intended to be a criticism of the Black Pearl. I think it looks great and I'm sure it performs impressively. I am just making the point that a kayak with the same performance on flat water could be made lighter.