After doing the P&P heads, oversize valves, headers, cams, mufflers, and a dyno tune, I am now considering getting the larger Throttle body. So is it really a needed thing considering the price, wondering if it will return enough peformance gain to make it worth while.
Second, which size would be best for my 2013 103 with 10.5:1 CR? How about the wye behind the TB? Seems to me that would also need to be changed to a bigger bore Then, if I did the change, would I need to change the injectors as well?
And lastly, would it need another dyno tune, or would it be close enough to simply run it with the current tune? Dyno as it is was about 96 HP 113 tq. Just wondering how big a difference it will make to go with the bigger TB or leave well enough alone, and if there is a thing of too much TB?

What cam are you running? I was thinking your numbers would have been a little higher. No expert. I've just been doing lots of reading on cams, heads and compression.

58 mm

I have the S&S 551 torque cams installed.

Yes the back plate will need to be changed if you go for the bigger TB. Now as far as the bigger TB goes it would have been best to consider that when thinking of the engine mods. To get the best results everything needs to work together so without knowing exactly what was done to the heads and how they were worked it's hard to say if a larger TB will actually help or hurt. Keep in mind bigger isn't always better and intake air is more about velocity than volume. Talk to the people that built the heads and they should be able to tell you exactly what TB will give you the best results considering cams, valve size, exhaust etc.... Most likely the 58mm will do better but I would do the research with the people that designed the heads or at least builders that have experience with your combo.

Ok. i'll ask the guy who built and flowed the heads. Dunn performance out of Washington state. He did a really nice job on them, and the documentation was awesome. They had a special type of seat grind as well, but I don't recall the name of the grind.

Dead on Ken.

I might add, your builder should be able to look your dyno graph, lay it over similar builds, and see if the throttle body is holding it back. On injectors, the dyno guy would have told you that you needed injectors. There are certain areas of the dyno fuel spread sheet that will max out if you are getting over 80% duty cycle on your injectors.

I was able to get a very prompt response from the shop that built the heads. They told me that the port to manifold Id is stock, so I should get the 55mm TB with a plenum that necks it back to the port Id. He said that larger ports would cause a stagnant spot in the ports.

I got almost the same #'s from a Woods TW222 cam on my 103 (112 ft lbs of torque) with the stock heads. No you don't need a 58mm TB the 103 works better with the stock 50mm TB and injectors. The 58mm TB and bigger injectors will give you some more top end but the extra fuel and air flow will be too much for the lower rpm range (2000 to 3000 rpms). If you do a bigger TB and injectors then you should get a dyno but IMO you should just leave the 50mm TB in there and ride, 113 ft lbs of torque out of a 103 makes for a nice ride. I've been there and done that and my 58mm TB and bigger injectors or on my island of misfit toys.