If a person enjoys themselves in their free time, they will be more productive at work.
A person will do better at work if they can relax and enjoy themselves when they are off work.
ーIf a person enjoys themselves in their free time, they will be more productive at work.
「余暇を充実して過ごすなら、仕事でもより生産的になれる」
to enjoy oneself in one's free time 「自由時間を楽しむ」で「充実した余暇を過ごす」ことを表現できます。
productive で「生産的な・実りの多い」
ーA person will do better at work if they can relax and enjoy themselves when they are off work.
「休みに充実して過ごせると、より良い仕事ができる」
to relax and enjoy oneself when one is off work「休日にリラックスして楽しむ」を使っても「充実した余暇を過ごす」ことを表現できます。
ご参考まで！
カナダ 
