I do believe "studied" and "had studied" are equally valid. Past subjunctive is ideal for expressing this contrary to fact wish, but it has two possible manifestations. "Had studied" (past subjunctive "had") works well, for sure. "Had" functions to express a contrary-to-fact, not place the wish in the past. Subjunctive mood transforms "I have studied" into "I wish I had studied". You can also create a wished for contrary-to-fact by using "studied", the past subjunctive of "study". Past subjunctive creates no time frame, so "studied" is equally correct.