not at all. Bruguera at RG was a different beast to the half-assed, non-peak effort he put into non-RG tournaments.
Many factors must coalesce for someone to win a grand slam. Even the likes of Federer, Nadal and Djokovic have occasionally needed some good fortune. The 1990s were of course much more varied in terms of surfaces and styles. Player depth, particularly clay court depth, increased significantly in the mid to late '90s. Timing and circumstances were crucial. Draws at slams could be very different in terms of difficulty for seeded players.
With that said, I totally refute the notion that Bruguera made "half-assed, non peak effort" at "non-RG tournaments," as if to explain his lack of winning elsewhere while propping up his approach as superior to others who perhaps compiled more well-rounded resumes without as many deep runs at RG.
Prior to his 1993 RG title, Bruguera had compiled a 5-4 record in Paris, having lost in the fourth round to Ronald Agenor in 1989, the round of 64 to Jonas Svensson in 1990 (Bruguera lost 6-0 in the fifth after leading two sets to love), the second round in 1991 to Omar Camporese (Bruguera retired with back injury while leading two sets to one; He said he felt something while training in between in first and second round matches), and in the first round in 1992 to Lendl 6-4, 6-2, 6-1. He played virtually the same spring clay court schedule leading up to Roland Garros in those years that he would for the remainder of his career.
In the lead-up to his 1993 RG title, Bruguera played in six tournaments -- Estoril, Barcelona, Monte Carlo, Madrid, Rome and the World Team Cup in Dusseldorf. Prior to Estoril, Bruguera lost two five setter Davis Cup matches on clay in Spain to Dutch opponents. I highly doubt that Bruguera made a "half-assed, non-peak effort" in front of his home country fans. Similarly, I also doubt that he made "half-assed, non-peak effort" in the Barcelona and Madrid finals, where he lost to Medvedev and Edberg, respectively. In between those Spanish events, Bruguera impressively won Monte Carlo, earning a rare win over Muster in the semis. Muster was the only player to win a set from Bruguera that week.
For someone apparently only interested in RG, it's curious that Bruguera chose to play in Rome and Dusseldorf in the two weeks preceding the 1993 event. Bruguera ran out of steam against Courier in Rome, but then won three straight matches in Dusseldorf, including a 6-1, 6-3 decision over Sampras. The Spaniard carried that momentum into Paris, where he further benefited from a very weak early round draw. In short, Bruguera was red hot coming in, got to conserve energy during the first week, and was able to maintain his nerve against Sampras, Medvedev and Courier down the stretch.
1994 followed a similar script, although Bruguera did not play in Rome that year. He lost to good opponents in Estoril (retired against a young Albert Costa), Barcelona (Krajicek), the Monte Carlo final (Medvedev) and the Madrid final (Muster). Bruguera won three of four matches in Dusseldorf on the eve of RG and again breezed through a very generous first week draw at RG before beating Rafter, Medvedev, Courier and Berasategui for the title. Courier, while still a formidable opponent, was not in tremendous form during the spring of '94. His path to the quarters that year featured three extremely unheralded opponents and another, Bjorkman, who was ranked 105th at the time. Contrast Courier's draw with Muster's and you again see how different things could be.
We could do similar exercises for Bruguera's 1995, 1996 and 1997 campaigns, but it would be redundant. The bottom line is that Bruguera was an excellent clay court player who maximized his brief window at RG thanks in part to some good fortune in terms of timing and draw circumstances, not because he utilized a unique strategy of making "half-assed, non-peak effort" at lead-up events.