If the Telefunken M15 is still calibrated according to german broadcast standard +6dBu at 514nWb/m flux you can expect a max. output no higher than ~15dBu peak with the hottest tapes, usually below.

So Fireface 800 can easily handle the level.

Quality-wise Fireface 800's AD-conversion is still top, more then sufficient, specially if you follow my advice:

Do the tranfer in 192kHz or at least 96kHz sample rate, in 24bit of course:
• Streaming services start to offer high bitrate "Master Quality" now.
• 15ips studio tapes contain signals well above 20kHz, which would not be recorded if you go for 44.1 or 48kHz SR.
• A lot of the digital processing, specially Equalizer's and Limiter's sound benefits from higher sample rate.

• For the transfer the playback head should be cleaned as often as possible.
• Get the machine aligned if you can't do this by yourself.
It's worth it, once aligned the M15 is extremly stable.

• I check, and if necessary align, the PB head's azimut to the individual tape that's about to be transferred.
This is not easy, I'd only advise to do so if you have reference tapes in good condition and an Oscilloscope to align back to standard afterwards.

The above is giving you more sound improvement than any new converter could.

If you really want to invest to achieve a theoretical 1‰ advantage:

I'd suggest an RME studio interface that supports RME's "Total Mix FX" and DigiCheck, like the Fireface UFX II.
You already have Total Mix and Digicheck now, once you start to use it you won't like to go without.

The monetary difference can be reduced or eliminated by selling the Fireface 800.