You need the right to tools for the job. Using FCP is an alternate way to work with the offspeed material.
For that kind of work we use, on location, one tape for 24p and a different one for offspeed material. Then we process it on our Panasonic Frame Rate Converter. From there you can go SDI to a NLE, HD-SDI to tape again or to a HD NLE via HD-SDI. Never have a single problem.
As for quality, is quite curious what you