Are filled 7200 drives still fast enough to play 4K BD rips?

Even the el-cheapo acoustic tuned 14TB 5400RPM Western Digital externals (WD140EDFZ) can maintain sequential speeds in excess of 100MiB/s when full (~ 110-200MiB/s). That's a much higher data rate than BD-ROM drives, so yes. Shouldn't struggle at all unless your files are extremely fragmented and the drive heads are thrashing or you are accessing many files simultaneously.