The difference is 110 degrees between the coldest and hotest cylinder.
A colleague of mine says that is a bit high for a fuel-injected system.
Is that right?

That's not right. It's not even wrong.

EGT is not of much value as an absolute measurement. That's what you
have TIT for. The sort of difference you have can easily be explained
by a slight variation of probe installation - if the #1 probe is a
little farther from the exhaust port than the #5, that can explain all
the difference.

The important question is this - do all the EGT's peak at the same fuel
flow? The thing to do is make incremental adjustments with the
mixture, and at each setting record fuel flow (0.1 gph increments are
good) and EGT on each jug. Then you generate a curve of EGT vs fuel
flow for each of the jugs. As long as they all peak together (meaning
at the same fuel flow), absolute differences mean little.

If they don't peak together, you can try swapping injectors around.
It's not strictly legal - but it's not actually illegal either. There
is no requirement to keep track of which injector goes to which jug
when they're cleaned, since in theory they are all the same diameter.
In practice the published tolerances are ridiculously wide, and half of
them don't meet spec anyway, but let's not confuse the issue with the
facts. If you "accidentally" got them reversed next time they were
cleaned, well, these things happen.

That used to happen a lot on my engines, until they strated running LOP
smoothly. Ever since then, I've kept track of which injector goes to
which jug. No rule against that either, you know.

Michael