The best flat field macro lens for 1:1 reproduction

Hello everybody, :)

I'm in the process of digitizing my film archives and I'm trying to get the best possible results.

I've been using my 5Ds with an adapter and a vintage slide duplicating rig, consisting of bellows and a film holder.

Something similar to this:

The hole "scanning" rig has to be carefully aligned, so that the camera sensor is perfectly parallel to the plane of the film to be copied.
This is not easily achievable with the camera mounted on tripod and the film placed on some light table.

The lens that I'm using is a Pentax SMC 100mm f/4 Macro.
The results are very good, but since I'm the OCD kind of guy, I always wonder If I could do better. :)

The Pentax SMC 100mm Macro is very sharp in the center with the film grain structure that is clear and defined.
In the corners the grain is still visible but is progressively getting mushier towards the extreme corners, where it is kind of OK and acceptable, but could be better.

I know that there are specialized lens that are made exactly for this type of work, like Rodenstock Apo Rodagon D, but they are very expensive and not easy to come by in my part of the world.

My question is: does any forum member have the first hand experience with Canon EF Macro lenses in this kind of application - 1:1 copy work that requires the best possible flat field performance?

My first bet would be the EF 100mm f/2.8 USM Macro (non L, since I don't need IS), but it is very hard to find any info on the net describing its flat field performance in the 1:1 range.

In other words - I'm not sure if (at f/8 aperture) the EF 100mm f/2.8 USM Macro will be a noticeable improvement over my Pentax 100mm macro?

The difference should really be worth it, since I would have to redesign my scanning rig from the ground up, because EF lenses can not be used with my bellows.
Thanks for the tip. :)
Yes, I know that, I've done the mirror thing long time ago.
It's Ok, but not practical for the high volume scanning, since you can easily knock things out of the pefect alignment easily if you just use a camera tripod and a light box.
I need a stable and sturdy setup, with easy and repeatable precise settings.
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