But if you want to keep the same cleats, then you can upgrade to Ultegra or Dura Ace. I run Ultegra pedals on my bikes as to me it is not worth the extra yen to save about the weight of a packet of gum. But I am not a weight weenie. But there is hardly any difference between the weight of 105s and Ultegra.
Like Ludwig said.. decide on the system you want to use first.
Personally, I like Speedplay pedals. I switched years ago and haven't thought about going back. (I have thought about buying SPD pedals to use the Shimano RT80 shoes - comfortable walking shoes are tempting.)
Weight is not an issue for me, but a pair of Speedplay Zeros (titanium) weigh in at 164 grams and cleats weigh and additional 140 grams. (The X series is lighter, but I'm not sure they sell those in Japan.)
Go to Ultegra. It's cheap(Not cheap actually) and performance is nothing different with Dura-ace/Mavic/Time/etc in my view. Just a bit difference like weight, but it's just nothing(50g? pay your money for more important parts) and you don't need to be sensitive with that.
The most important part of the bike are contact points either between yourself and the bike or the bike and the road. Thats wheels, tires, pedals, cranks, chainrings and cassettes freewheels and so on. They are all moving parts that take stresses while riding.
There maybe little difference between the weight of say Ultergra and DuraAce pedals but the bearings, axels and other working parts will be very different, with either sealed bearings or ceramic offering much less roling resistance and in the grand scheme of things this is very important, it's not just a weight thing.
FarEast, Maybe those reason will be very important for serious people someone like pro/elite/racer. But in my opinion, I used PD-7800 & PD-6620-G, I couldn't get any difference(100$ worth?) between of them. Maybe it's because I'm junior cyclist and still don't know about the bike. But, if I have to choose the component between top and middle range, I'll always prefer to have middle range. (Jesus, top component is too expensive!!) Anyway, it was not good manner to mention specific brand name and model. I apologize this.